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From its completion in 1950 until Rakuten began renovating the facility in 2004, Miyagi Stadium accommodated approximately 28,000 people. This relatively low capacity factored into the decision to not play the 1974 Japan Series in the Stadium. Initially, Rakuten's announced renovation design revealed that the ballpark's seating capacity was planned to be 23,000 by the start of the 2005 season and 28,000 by the following spring. After the two-stage renovation, however, its seating capacity was only 20,000 and 23,000, respectively. Almost yearly redevelopment caused the number to continually change. It was reduced to 22,187 in 2008 and 22,098 in 2009, but expanded to 23,026 in 2010. By the start of 2013, capacity was increased to 23,451 and again to 23,466 by April. Rakuten was cautious about rapidly expanding the stands because of the stadium's location in a public park and Sendai's population of approximately 1 million people, a relatively small market for a baseball franchise in Japan. Furthermore, the stadium is located in a public park subject Japan's City Park Law, an ordinance dictating how much park space can be occupied by buildings, making it difficult to freely expand the facility.

After the Eagles advanced to the Second Stage of the 2009 Climax Series, NPB became concerned about Miyagi Stadium's low seating capacity. Compared to the other stadiums in NPB that all held at least 30,000 people, Rakuten's approximate capacity of 22,000 was significantly lower. Though they were eliminated in the Second Stage, if Rakuten had won and advanced to the Japan Series, NPB claimed that there would have been a ¥100 million profit difference per game held at Miyagi Stadium compared to the Sapporo Dome. This loss of profit would have not only affected Rakuten, but also the Central League team they played and NPB, as proceeds from Japan Series attendance are split among the three parties. NPB's concern led to some speculation that if the Eagles ever advanced to a Japan Series, the games would not be held in Miyagi Stadium. Instead of increasing Miyagi Stadium's seating capacity, Rakuten suggested that they could increase ticket prices to make up the difference. Team owners and NPB, however, requested that they increase capacity to at least 28,000, citing Rakuten's original pledge to do so when they were first awarded a team.

Mid-season in 2013, the Eagles were in first place in the Pacific League and selling out games played at Miyagi Stadium. To help meet fan demand, Rakuten erected two temporary seating sections in the ballpark. The resulting 936-seat section along the third-base line and the 1,249-seat section on the left field lawn increased Miyagi's capacity from 23,466 to 25,651. The seats opened to fans that September. However, NPB had been pushing Rakuten to increase the stadium's capacity to at least 28,000, specifically if the stadium needed to host Japan Series games, therefore Rakuten continued to add more temporary seating in early October as the postseason neared. Before it hosted its first Final Stage during the 2013 Climax Series, more temporary and standing seats brought the stadium's capacity to 26,965. Finally, the addition of 1,155 standing seats before the 2013 Japan Series brought Miyagi Stadium's capacity to 28,120.

Following the Japan Series, the temporary stands down the third-base line and in left field were dismantled. In the offseason, another temporary seating section was constructed in left field, bringing capacity to 25,717. Five months later, work was completed on new, permanent seating in the former location of the temporary stands down the third-base line. The 3,019-seat section pushed the stadium's capacity over 28,000 once again. The last large-scale renovation of Miyagi Stadium occurred in the 2015–16 offseason that dramatically increased its capacity one last time. The project removed the temporary stands in middle-left field that could seat approximately 4,000 spectators and instead constructed a park capable of accommodating around 7,000, bringing Rakuten Seimei Park to its current capacity of 30,502 people.

From its opening in 1950 until 2004, Miyagi Stadium's playing field featured a natural turf outfield and a dirt infield; the distances from home plate to the outfield wall were symmetrical: 91.4 metres (300 ft) to the foul poles and 122 metres (400 ft) to the center field wall. Rakuten's first renovation of the stadium prior to the 2005 season dramatically changed the field. The foul poles were pushed back to 101.5 metres (333 ft), the longest in NPB at the time, and also extended the left- and right-center field walls to 117 metres (384 ft). Two seating sections were also constructed on the field along the first and third baselines, greatly reducing the stadium's foul territory. Additionally, this renovation converted the natural turf and dirt field playing field to an artificial surface using FieldTurf. This artificial turf had to be replaced once after the 2007 season. The outfield dimensions changed one last time during the 2012–13 offseason when two new seating sections were built in left- and right-center fields that protruded from the existing outfield seating. These new sections effectively created a "lucky zone" on the field, a Japanese term that describes moving an outfield wall to make hitting home runs easier. The additions reshaped Rakuten Seimei Park Miyagi's field to its current dimensions of 100.1 metres (328 ft) to each foul pole and 116 metres (381 ft) and 122 metres to the outfield walls in left/right-center and center fields, respectively. Miyagi Stadium's artificial turf was eventually removed and natural grass was again introduced prior to the start of the 2016 season. Instead of the black soil used in Koshien Stadium's infield, Miyagi Stadium's infield uses the red clay featured in MLB ballparks. It remains the only natural turf field in the Pacific League.

Miyagi Stadium's first, simple outfield scoreboard only displayed the game's line score. It was demolished and replaced with a larger, partially-electric scoreboard prior to the start of the 1973 season. In addition to the new electric inning-by-inning scoring and indicator lights, the new board featured handwritten batting lineups and umpire listings as well as a clock. This scoreboard was used until 2004, when it was demolished on December 6 during Rakuten's renovation of Miyagi Stadium. The stadium's third scoreboard iteration was installed in March 2005, just before the start of the Eagles' inaugural season. The 25.5 m × 10.5 m (84 ft × 34 ft) board was created by Toshiba and its layout was similar to its predecessor; the lineups and line score, however, were presented via electric display along with the batter's average and pitching speed. New to the scoreboard was an 8 m × 6 m (26 ft × 20 ft) full-color LED screen.

During the 2009–2010 offseason, an Aurora Vision video board developed by Mitsubishi Electric was installed in right-center field. The 20.6 m × 16.3 m (68 ft × 53 ft) high-definition LED screen is one of the largest in an outdoor stadium in Japan and was seven times larger than the screen embedded on the scoreboard at the time. The screen was erected to the right of the scoreboard to block the view of residents of an apartment building located behind the stadium's right-center field bleachers. Six years later, the center-field scoreboard was renovated for a third and final time; it was converted to a Panasonic full-LED video screen capable of displaying additional team and player statistics.

When the newly renovated Miyagi Stadium opened in 2005, Rakuten Mountain, the left-center field seating area, was a turf lawn until a temporary seating section was erected there in 2013. During the 2015–16 offseason, the temporary stands were removed and the area was redeveloped into a "park within a park" to further push Rakuten's "ball park concept". Named Smile Glico Park for sponsor Ezaki Glico, the resulting 4,000 m (43,000 sq ft) recreation area accommodates approximately 7,000 people and includes amusement attractions and game seating. The highlight of the park is a 16 gondola, 36-metre-tall ferris wheel from which fans can watch the game that was requisitioned from the closed Sendai Hi-Land amusement park. A merry-go-round and a playground were added to the park, as well as other temporary attractions such as a climbing wall and a double-decker bus. Fans have access to the amusement area with purchase of a ticket to an Eagles game, however individual tickets to Smile Glico Park are also sold separately and on non-game days. In 2020, a café building was converted into a small, 4-room, Eagles-themed hotel named Rakuten STAY x EAGLES. in the park is also open to reservations for both game and non-game days.

Rakuten opened a thrill ride at the top of the stands on the third base side in 2018. The ride, dubbed the "Eagle Bridge", consisted of two beams suspended 20 m (66 ft) above the ground, one to walk across and another with a swing affixed to the end. In addition to being available during game days, the attraction was also operated on non-game days when Smile Glico Park was open despite not being located within the Glico Park. Eagle Bridge was dismantled before the 2020 season.

Miyagi Stadium is the third-oldest of twelve NPB stadiums and the oldest in the Pacific League. Other than the addition of lights in 1973, the stadium's only other significant renovation was the reconstruction of the stands directly behind home plate to include individual seats in 1984. By 2004, the 54-year-old ballpark still featured simple bench seating down the first and third baselines, and the outfield section was a lawn with no formal seating. The playing field had a dirt infield and short distances down the right and left field foul lines to the outfield wall. The outfield scoreboard installed prior to the 1973 season still displayed team batting lineups via old-fashioned panels with players’ names hand-painted on them. Renovations to the aging facility were an important part of the selection process when NPB was in talks with Livedoor and Rakuten to form new teams to be based in Miyagi Stadium. Prior to the selection hearings, professional baseball sent the selection subcommittee to the stadium to assess its state. During the hearings, the two applicants discussed their plans to renovate the infield seating, add outfield seating, increase capacity, upgrade concessions, replace the scoreboard, and extend the foul lines, among other topics. At this time, general contractors Takenaka Corporation and Kajima had also put together proposals to convert Miyagi Stadium into a domed facility costing ¥23.5 billion and ¥10 billion, respectively.

Upon being selected as the new NPB team in November 2004, Rakuten president Hiroshi Mikitani traveled to Miyagi Stadium to sign a basic agreement with Shiro Asano, governor of Miyagi Prefecture, officially securing the ballpark as the Eagles' home field. There, he revealed the redevelopment details alongside an artist's rendition of the stadium post-renovation. Rakuten initially planned to spend approximately ¥3 billion to modernize the stadium in two phases. The first phase would bring the ballpark's seating capacity to 23,000 by the following spring and phase two would increase the capacity to 28,000 by the spring of 2006. Extra seating would be placed in foul territory to bring fans closer to the field and box and VIP seating would be added. Additionally, the first- and third-base foul lines would be extended from 91.4 meters to 99.7 meters, a video screen would be installed on a new fully-electronic scoreboard, and the field would possibly be replaced with artificial turf. Mikitani compared the stadium's new plan to that of a Major League Baseball (MLB) stadium.

The first phase of Rakuten's renovation of Miyagi Stadium was completed entirely during the 2004–2005 offseason. Kajima Corporation was hired for the project and demolition started in December, 2004. In the first week of the project, the infield seats were removed and the scoreboard was demolished; a new, fully-electronic board was installed in its place before the start of the season, . In addition to replacing all of the infield seats, new seating sections were also constructed. In the infield, two sections that cut into foul territory down the first and third baselines were added. Additionally, an area was dug out behind the backstop and seats were installed to allow for spectators to view the game from a lower perspective. Since the stadium only had lawn seating in the outfield prior to the renovation, new permanent seating needed to be built. On the far left- and right-sides of the outfield, two new sections were constructed, while grass seating was maintained on either side of the new scoreboard. Beyond the left field seating, an 8-metre (26 ft) high mound dubbed "Rakuten Mountain" was created using leftover dirt excavated during construction. The sloped hill connected the area behind the stadium to the grass seating area in left field.

Around the front of the stadium, a large, concrete frame was constructed that expanded the concourses, allowing for restrooms and shops to be upgraded and added. In addition to stadium infrastructure, baseball facilities and amenities were also modified. The baseball field's outfield wings were extended from 91.4 to 101.5 metres (300 to 333 ft), the largest in NPB at the time, and its natural turf was replaced with artificial FieldTurf. A facility housing the pitchers' bullpens was added on the outside of the stadium along its third base side, allowing fans to watch players warm up. Construction concluded on March 20 and the Eagles held their first game at the stadium on April 1, 2005.

Work on the second phase of the project started the following offseason in October 2005. The concrete frame constructed around the front of the stadium during the first phase was expanded upon and completed, resulting in a five-story structure that contained a food court, box seats, the TV and radio booth, the press box, and a premium lounge. Additional infield seating was added via a new section erected atop the building that houses the bullpens on the third base side and box seats and an enclosed lounge with a video screen above it built along the first base side. Four other adjoining buildings were built adjacent to the stadium on the first base side. These facilities housed the team's offices and clubhouse, two indoor practice areas, and a parking garage. With phase two concluded, Rakuten had spent ¥7 billion in total on the two-year modernization project.

Prior to the 2007 season, the first ribbon display at a baseball stadium in Japan was added to Miyagi Stadium's backstop. Two more ribbons were added the following season to the outfield seating areas and the right field lawn section was converted to a group seating area that provided tables for groups up to five people. Upgrades continued in 2009 and 2010 when the sunken backstop seats were enclosed to help create the new "Prestige" section, and the large video board was installed in right field, respectively. Renovation plans were halted in 2011 when repairing the damage caused by the 2011 Tōhoku earthquake took precedent.

Following the 2012 season, Rakuten continued its redevelopment of Miyagi Stadium and again focused on implementing their "ball park concept". The box seats along the first base side were expanded farther toward the outfield and included a party deck. The bigger change, however, came with the addition of the new E-Wing seating sections. These two sections were built in right- and left-center fields protruding from the existing outfield seating. The construction of these seats changed the shape of the outfield and decreased the distances to the wall; both distances down the first and third baselines were reduced from 101.5 to 100.1 metres (333 to 328 ft), and middle left and right fields were shortened from 117 to 116 metres (384 to 381 ft), effectively creating a "lucky zone", the term used to describe moving an outfield wall to make hitting home runs easier. Along with the E-Wings, a new outfield wall was also constructed. This wall was shorter than the previous (2.8 to 2.5 metres (9.2 to 8.2 ft)) and featured padding for outfielder players.

Rakuten removed much of the temporary seating added throughout the year during the Eagles' championship in 2013 and replaced it with a large, permanent seating area along the third base line. In addition to traditional seating, the section also featured seats with tables, box seats, party decks, and standing seats. It was completed in August 2014, with a 6 m × 35 m (20 ft × 115 ft) LED screen added to the top the following September. The following offseason, a new entrance gate and standalone Eagles' merchandise store were built in the plaza in front of the stadium and a geodesic dome was erected on its third base side. A restaurant for season ticket holders was also constructed in the ballpark.

The last large-scale renovation of Miyagi Stadium occurred during the 2015–16 offseason. The ¥3 billion project converted the field from artificial turf back to natural grass, updated the scoreboard to a full LED screen, redeveloped the center-left stands into an amusement park, and made minor alterations to several seating sections. Rakuten first began testing the feasibility of maintaining a natural turf field in Sendai's cold climate by collecting data on grass installed on a practice area next to the stadium in 2014. During the last major seating alteration following the 2018 season, a four-story tower was constructed and the first base side infield seats were upgraded. The new 25 m (82 ft)-tall tower, dubbed "Eagles Tower", sits just beyond the foul pole on the first base side and can hold up to 103 spectators.

Miyagi Stadium's name had remained unchanged since it opened in 1950. However, before the start of Rakuten's first season in Miyagi Stadium, the team and the prefecture agreed to sell the naming rights to the stadium to help pay for team management costs. The agreement stipulated that the rights would be renegotiated every three years and the money from the each contract would be split between the team and the prefecture at a three-to-one ratio, respectively. Additionally, any new stadium name must include the word "Miyagi". The first contract was awarded to the only bidder, staffing firm Fullcast, and in early 2005, the ballpark's name was changed to "Fullcast Stadium Miyagi" for ¥600 million. However, in August 2007, the Tokyo Labor Bureau ordered Fullcast to suspend operations after it found that the company had violated labor law. After this incident, Fullcast's contract was terminated several months early, and its name was removed from the ballpark.

After Fullcast's contract was cancelled in 2007, Miyagi Prefecture began soliciting applications for a new naming sponsor that same year. Nippon Paper Industries committed to a ¥750 million contract to rename Miyagi Stadium "Nippon Paper Kleenex Stadium Miyagi" beginning on January 1, 2008. However, just before the start of the season, it was revealed that Nippon Paper had misled the public about how much recycled paper was being used in their products. In the wake of the news, the contract wasn't terminated, however, it was agreed that the company's name would be dropped from the stadium leaving "Kleenex Stadium Miyagi". Near the end of the three-year contract period, all parties agreed to renew for another three years, however the price of the naming rights was lowered to ¥600 million. The company's name was also added back into the stadium's official name, making it "Nippon Paper Kleenex Stadium Miyagi".

In 2013, Nippon Paper decided not to renew its contract to purchase Miyagi Stadium's naming rights for a third time. Two companies applied after the prefecture opened applications, and Rakuten, the baseball team's parent company, was awarded the contract for ¥603 million. The ballpark was named "Rakuten Kobo Stadium Miyagi" from 2014 through 2016 to promote the company's ebook and e-reader subsidiary, Kobo Inc. When Rakuten renewed the contract at the end of 2016, the name was modified slightly to "Kobo Park Miyagi". The word "park" was inserted into the stadium's name to help convey the team's new "ball park concept" that promoted the facility as not only a place to watch a baseball game, but also as an entertainment destination for people of all ages. One year later and in the middle of the contract, Rakuten again changed the stadium's name, the third time in five years. The new name, "Rakuten Seimei Park Miyagi", was chosen to raise the profile of Rakuten's life insurance business. After this, fans became frustrated with the frequency of the changes. Rakuten renewed their contract a third time at the end of 2019 for the same amount as the previous contracts, ¥603 million. After keeping the name for five years, Rakuten changed the stadium's name again starting in 2023 with their fourth three-year, ¥603 million naming rights contract; the stadium's current name, "Rakuten Mobile Park Miyagi", takes its name from the company's mobile carrier subsidiary.

Miyaginohara Station on JR East's Senseki Line is the closest train station to Rakuten Seimei Park. Nicknamed "baseball station", it was repainted crimson red in 2005 to celebrate the founding of the team. Additionally, atop the station's entrance sits a large Eagles helmet, it features images of the team's mascots, and trains play a clip from the team's song "Habatake Rakuten Eagles" as their departure melody. Tsutsujigaoka Station, the next station west of Miyaginohara on the Senseki Line, also brings passengers to within walking distance of the stadium. Sendai Subway's Tōzai Line can also be used to get to Miyagi Stadium, with the line's Yakushido and Rembo Stations both bringing passengers to within a fifteen-minute walk. For fans arriving to Sendai Station via various lines, including the Sendai Subway Namboku Line, the Shinkansen, or the Sendai Airport Line, Rakuten operates a shuttle bus to and from the stadium from the station's east exit bus stop platform 76.

In addition to rail service, several bus options also provide transportation to the stadium. Locally, Rakuten Seimei Park is served by Sendai City Bus routes 230, 233, and 308. Some intercity bus routes operated by JR Bus Tōhoku, Aizu Bus, and Fukushima Transportation throughout the Tōhoku region also provide direct-to-stadium options on game days. For fans not using public transportation, the stadium is accessible by car via the Tōhoku Expressway (E4) on the west and Sendai-Tōbu Road (E6) on the east. The stadium has an on-site and an off-site parking lot as well as on-site parking for bikes and motorcycles.

• Lists of stadiums
• List of stadiums in Japan

Wikimedia Commons has media related to Miyagi Baseball Stadium.

• Official website (in Japanese)
• Rakuten Seimei Park Miyagi seating chart (in Japanese)