While the Church in Norway initially had refused to recognise Haakon as King of Norway, it had largely turned to support his claim to the throne by the 1223 meeting, although later disagreements occurred. Despite being the undisputed ruler of Norway after 1240, Haakon was still not approved as king by the pope due to his illegitimate birth. He nonetheless had a strong personal desire to be approved fully as a European king. Several papal commissions were appointed to investigate the matter, and Haakon declared his legitimate son Haakon the Young his successor instead of an older living illegitimate son. Although Haakon had children with his mistress Kanga the Young prior to his marriage with Margrete, it was his children with Margrete who were designated as his successors in accordance with a papal recognition. The Catholic principle of legitimacy was thus established in the Norwegian order of succession, although Haakon's new law still maintained that illegitimate children could be designated as successor in the absence of any legitimate children or grandchildren—contrary to Catholic principles. While his strong position allowed him to set boundaries to the Church's political influence, he was on the other hand prepared to give the Church much autonomy in internal affairs and relations with the rural society.

Haakon also attempted to strengthen his ties with the papacy by taking a vow to go on Crusade. In 1241 he converted this into a vow of waging war against pagan peoples in the north in light of the Mongol invasion of Europe. When a group of Karelians ("Bjarmians") had been forced westwards by the Mongols, Haakon allowed them to stay in the area surrounding the Malangen fjord and had them Christianized—something that would please the papacy. Later, in 1248, Louis IX of France proposed (by Matthew Paris as messenger) to Haakon to join him for a Crusade, with Haakon as commander of the fleet, but Haakon declined. While Haakon had been unsuccessful in gaining the recognition of Pope Gregory IX, he quickly gained the support from Pope Innocent IV who sought alliances in his struggle with Holy Roman Emperor Frederick II. Haakon finally achieved royal recognition by Pope Innocent in 1246, and Cardinal William of Sabina was sent to Bergen and crowned Haakon in 1247.

After consolidating his position in 1240, Haakon focused on displaying the supremacy of the kingship, influenced by the increasingly closer contact with European culture. He built several monumental royal buildings, primarily in the royal estate in Bergen, where he built a European-style stone palace. He used a grand fleet with stately royal ships when meeting with other Scandinavian rulers, and actively sent letters and gifts to other European rulers; his most far-reaching contact was achieved when he sent gyrfalcons with an embassy to the sultan of Tunis.

The royal court in Bergen also started importing and translating the first true European literature that became available to a wider Norwegian audience. The literature which was popular then was heroic-romantic literature derived from the French and, in turn, English courts, notably chansons de geste around Charlemagne (the Matter of France) and tales of King Arthur (the Matter of Britain). The first work that was translated into Old Norse was reportedly the Arthurian romantic story Tristan and Iseult, which was finished in 1226 after orders from the young and newly-wed Haakon. Haakon's programme seems to have been the spark for the emergence of a new Norse genre of chivalric sagas.

Haakon also had the popular religious text Visio Tnugdali translated into Old Norse as Duggals leiðsla. The literature also appealed to women, and both Haakon's wife Margrete and his daughter Kristina owned richly illustrated psalters.

Haakon also initiated legal reforms which were crucial for the development of justice in Norway. Haakon's "New Law", written around 1260, was a breakthrough for both the idea and practice of public justice, as opposed to the traditional Norwegian customs of feuds and revenge. The influence of the reforms is also apparent in Haakon's Konungs skuggsjá ("King's Mirror"), an educational text intended for his son Magnus, which was probably written in cooperation with the royal court in the mid-1250s.

Relations were hostile with both Sweden and Denmark from the start of his reign. During Haakon's rivalry with Earl Skule, Skule sought the support of Valdemar II of Denmark, but any aid was made impossible after Valdemar's capture by one of his vassals. Since the Danes wanted overlordship of Norway and supported the Guelphs (those supporting the Pope over the Holy Roman Emperor), Haakon in turn sought closer ties with the Ghibelline Emperor Frederick II, who sent ambassadors to Norway. As Haakon had gained a powerful reputation due to the strength of his fleet, other European rulers wanted to benefit from his friendship. Despite the struggle between the Pope and the Emperor, Haakon was able to maintain friendships with both. According to an English chronicler, the Pope wanted Haakon to become Holy Roman Emperor. It has been suggested that Haakon hesitated to leave Norway due to the Mongol threat.

Haakon pursued a foreign policy that was active in all directions (although foremost to the west and south-east). In the north-east, the relationship with Novgorod had been tense due to a dispute over the right to tax the Sami people, as well as raiding from both Norwegian and Karelian sides. Eventually, the Mongol invasion of Rus' drove Prince Alexander Nevsky to negotiations with Haakon that likely strengthened Norwegian control of Troms and Finnmark. An embassy from Novgorod one time asked for the hand of Haakon's daughter Christina, but Haakon refused due to the Mongol threat. With Norwegian ships from the port of Elven were active in the waters south of Sweden and into the Baltic Sea, Norway increasingly relied on Baltic grain from Lübeck. This trade was halted in the late 1240s by the plundering of Norwegian ships in Danish seas by ships from Lübeck. In 1250, Haakon made a peace and trade agreement with Lübeck, which eventually also opened the city of Bergen to the Hanseatic League. During the conflict, Haakon had reportedly been offered control over the city by Emperor Frederick II. In any case, Haakon's policy regarding Northern German ports largely derived from his strategy of attempting to exploit the internal turmoil in Denmark after the death of King Valdemar II in 1241.

In Scandinavia, Haakon regularly met with neighbouring rulers in the border-area around Elven from the late 1240s through the 1250s. He sent grand fleets as embassies; some reportedly numbered over 300 ships. Haakon also reconciled with the Swedes when he his son Haakon the Young married Rikissa, a daughter of Swedish leader Earl Birger. Haakon sought to expand his kingdom southwards of Elven into the Danish province of Halland. He thus looked for alliance with the Swedes, as well as ties with opponents of the ruling line of monarchs of Denmark. In 1249, Haakon allied with Earl Birger for a joint Swedish-Norwegian invasion into Halland and Scania, but the agreement was eventually abandoned by the Swedes (see Treaty of Lödöse). Haakon claimed Halland in 1253, and finally invaded the province on his own in 1256, demanding it as compensation for the looting of Norwegian ships in Danish seas. But he renounced this claim in a 1257 peace agreement with Christopher I of Denmark. Haakon thereafter negotiated a marriage between his only remaining son, Magnus, and Christopher's niece Ingeborg. Haakon's Nordic policies initiated the build-up to the later personal unions (called the Kalmar Union), that in the end had dire consequences for Norway as it did not have the economic and military resources to persevere and maintain Haakon's aggressive policies.

More distantly, Haakon sought an alliance with Alfonso X of Castile (a potential next Holy Roman emperor) chiefly as it would guarantee new supplies of grain to Castile in light of rising prices in England, and possibly giving access to Baltic grain through Norwegian control of Lübeck. Alfonso in turn sought to expand his influence in Northern Europe, as well as to gain Norwegian naval assistance for the campaign or crusade he had proposed in Morocco (seeing that the Iberian Moors received backing overseas from North Africa). Haakon could thus potentially also fulfill his papal vow of crusade, although he likely did not intend to. He sent an embassy to Castile in 1255. A Castilian ambassador to Norway returned with the embassy, and proposed to establish the "strongest ties of friendship" with Haakon. At the request of Alfonso, Haakon sent his daughter Christina to Castile to marry one of Alfonso's brothers. However, Christina died four years later without children, which marked the effective end of the short-lived alliance, and the proposed crusade fell into the blue.

Haakon employed an active and aggressive foreign policy to strengthen Norwegian ties in the west. His policy relied on friendship and trade with the King of England; the first known Norwegian trade agreements were made with England in the years 1217–23 (England's first commercial treaties were also made with Norway), and the friendship with Henry III of England was a cornerstone of Haakon's foreign policy. As they had become kings around the same time, Haakon wrote to Henry in 1224 that he wished they could maintain the friendship that had existed between their fathers. Haakon sought to defend the Norwegian sovereignty over islands in the west, namely the Hebrides and Man (under the Kingdom of Mann and the Isles), Shetland and Orkney (under the Earldom of Orkney), and the Faroe Islands. Further, the Norse community in Greenland agreed to submit to the Norwegian crown in 1261, and in 1262 Haakon achieved one of his long-standing ambitions when he incorporated Iceland into his kingdom by exploiting the island's internal conflicts in his favour. The dependency on Norwegian maritime trade and their subordination to the Nidaros ecclesiastical province were some of the key reasons which allowed Haakon to assert sovereignty over the islands. The Norwegian kingdom was at the largest it has ever been by the end of Haakon's reign.

Norwegian control over the Faroe Islands and Shetland was strong due to the importance of Bergen as a trading centre, while Orkney, the Hebrides, and Man had more natural ties with the Scottish mainland. Although traditionally having had ties with the community of Norse settlers in northern Scotland, Scottish rulers had increasingly asserted their sovereignty over the entire mainland. Haakon had at the same time gained stronger control of the Hebrides and Man than any Norwegian ruler since Magnus Barefoot. As part of a new development the Scottish king Alexander II claimed the Hebrides and asked to buy the islands from Norway, but Haakon staunchly rejected the proposals. Following Alexander II's death, his son Alexander III continued and stepped up his father's policy by sending an embassy to Norway in 1261, and thereafter attacking the Hebrides.

In 1263, the dispute with the Scottish king over the Hebrides induced Haakon to undertake an expedition to the islands. Haakon learned in 1262 that Scottish nobles had raided the Hebrides and that Alexander III planned to conquer the islands. In 1263 Haakon mounted an expedition with his formidable leidang fleet of at least 120 ships. He had become accustomed to negotiating while backed by an intimidating fleet. The fleet left Bergen in July, and reached Shetland and Orkney in August, where they were joined by chieftains from the Hebrides and Man. Alexander started negotiations after Norwegian landings on the Scottish mainland, but the Scots purposely prolonged the talks. The Scots waited until September and October for weather that would be trouble for Haakon's fleet. A Scottish force met a smaller Norwegian force at the Battle of Largs (2 October). Although the battle was inconclusive, Haakon withdrew to Orkney for the winter. A delegation of Irish kings invited Haakon to become the High King of Ireland and expel the Anglo-Norman settlers in Ireland, but this was apparently rejected against Haakon's wish.

Haakon over-wintered at the Bishop's Palace in Kirkwall, Orkney, with plans to resume his campaign the next year. During his stay in Kirkwall he however fell ill, and died in the early hours of 16 December 1263. Haakon was buried in the St Magnus Cathedral in Kirkwall for the winter; in spring, his body was exhumed and taken back to Norway, where he was buried in the Old Cathedral in his capital Bergen. Centuries later, in 1531, the cathedral was demolished by the commander of Bergenhus, Eske Bille, for military purposes in connection with the Protestant Reformation, and the graves of Haakon and other Norwegian kings buried there might have been destroyed in the process or moved to another location.

Norwegian historians have held differing views on Haakon's reign. In the 19th century, P. A. Munch portrayed Haakon as a mighty, almost flawless ruler, which in turn influenced Henrik Ibsen in his 1863 play The Pretenders. In the early 20th century, poet Hans E. Kinck countered and viewed Haakon as an insignificant king subordinated to forces outside of his control, a view which influenced historians such as Halvdan Koht and Edvard Bull, Sr. Haakon has often been compared with Skule Bårdsson, and historians have taken sides in the old conflict. While Munch saw Skule as a traitor to the rightful Norwegian king, Koht viewed Skule as a heroic figure. On more sketchy grounds, Kinck praised Skule as representing the original and dying Norse culture, and Haakon as a superficial emulator of foreign culture. Since the 1960s, historians including Narve Bjørgo, Per Sveaas Andersen, Knut Helle, Svein Haga, and Kåre Lunden have in turn professed a reaction against Koht's view. According to Sverre Bagge, modern historians tend to follow Koht when it comes to see Skule's rebellion as a last desperate attempt to stop Haakon from encroaching on Skule's power, but lean closer to Munch's overall evaluation of the two men.

Knut Helle interprets the saga to leave an impression of Skule as a skilled warrior and politician, while noting that the author of the saga purposely created a diffuse image of his role in the conflict with Haakon. On the other hand, Helle notes that Skule was outmaneuvered with relative ease by Haakon's supporters in the years immediately after 1217, and that this may suggest some limited abilities. While neither giving a clear picture of Haakon, Helle maintains that Haakon "obviously" learned to master the political game in his early years. He interprets Haakon as an independent and strong-willed ruler to whom he assigns a "significant personal responsibility" for the policies pursued during his reign: notably regarding the internal consolidation of the kingship, the orientation towards European culture and the aggressive foreign policy. In his article in Norsk biografisk leksikon, Knut Helle acknowledges that Haakon was empowered by the strong institutional position of the kingship at the end of his reign (which he had developed himself), and that his policies were not always successful. Helle nonetheless recognises the substantial political abilities and powerful determination Haakon must have had in order to progress from the difficult position in which he started his reign.

Haakon had three illegitimate children with his mistress Kanga the Younger av Folkindberg (who is only known by name) (1198–1225), before 1225. They were:

• Sigurd (died 1252).
• Cecilia (died 1248). Married lendmann Gregorius Andresson, a nephew of the last Bagler king Philip Simonsson in 1241. Widowed in 1246, she married Harald Olafsson, King of Mann and the Isles in 1248. They both drowned the same year on the return voyage to Great Britain.
• Kone (c. 1225 -), married Toralde Gunnarsson Hvite, until Gulsvik (Buskerud, c. 1220 – Gulsvik, Flå, Buskerud, d. 1260), mentioned on a 1258 document, and had issue.

Haakon married Margrete Skulesdatter on 25 May 1225, daughter of his rival Earl Skule Bårdsson. Their children were:

1. Olav (born 1226). Died in infancy.
2. Haakon the Young (1232–1257). Married Rikissa Birgersdotter, daughter of the Swedish statesman Earl Birger in 1251. Appointed king and co-ruler by his father in 1240, but predeceased his father.
3. Christina (1234–1262). Married Infante Philip of Castile, brother of Alfonso X of Castile in 1258. Died childless.
4. Magnus VI of Norway (1238–1280). Married Ingeborg, daughter of Eric IV of Denmark in 1261. Appointed king and co-ruler after the death of Haakon the Young. Succeeded his father as King of Norway.

Håkon and Kristin were the mascots of the 1994 Winter Olympics. Håkon is named after Haakon IV of Norway and Kristin after Christina of Norway.

In The Last King (2016), the infant Håkon IV is portrayed by Jonathan Oskar Dahlgren.

Wikimedia Commons has media related to Haakon IV of Norway.

• The Norwegian account of Haco's expedition against Scotland, A.D. MCCLXIII at Project Gutenberg
• "Haakon" . Encyclopædia Britannica (11th ed.). 1911.