Mahopac (/ˈmeɪəˌpæk/ or /ˌməˈhoʊpæk/) is a hamlet (and census-designated place) in the town of Carmel in Putnam County, New York, United States. Also known as Lake Mahopac, the exurb is located some 47 miles (76 km) north of New York City, on US Route 6 at the county's southern central border with Westchester County. As of the 2020 census, the population was 8,933.

Mahopac was originally inhabited by the Wappinger people, an Algonquian tribe. The hamlet's land was part of a huge tract encompassing all of today's Putnam County patented in 1697 by Adolphus Philipse, son of a wealthy Anglo-Dutch gentryman, known as the Philipse Patent. During the French and Indian War, Wappingers throughout Putnam County traveled north to Massachusetts to fight for the British.

When the British Crown refused to return their land after the war, most Wappingers abandoned the area, concentrating in Stockbridge, Massachusetts before relocating with other displaced Native Americans elsewhere. Farmers and their families migrated to Mahopac from as far away as Cape Cod and rented land from the Philipse family. Wheelwrights and blacksmiths set up shops to assist the tenant farmers.

Although no battles were fought in Mahopac during the American Revolution, the area was strategically important due to its location. With troop encampments in nearby Patterson, Yorktown, West Point, and Danbury, Connecticut, it was a cross-roads between key Colonial garrisons. Soldiers were stationed in Mahopac Falls to guard the Red Mills, an important center for grinding grain and storing flour for the American troops.

Upon Colonial victory in the Revolution, the Tory-sympathizing Philipse family lost its claim to the land, which was then resold to farmers and speculators by New York State. After the incorporation of Putnam County in 1812 the Mahopac area grew steadily. By the middle-19th century the hamlet had become a summer resort community. The New York and Harlem Railroad brought vacationers north from New York City to Croton Falls. Hotels would often have competing races of decorated horse-drawn coaches bringing passengers from the train to Lake Mahopac. After the Civil War a direct rail spur was laid, creating boom times for the village.

The locale remained primarily a summer resort until after World War II, when nearby highways such as the Taconic State and Saw Mill River parkways began to make travel by automobile convenient. With the passing of the New York Central Putnam Division's last passenger service to Mahopac in 1959, the hamlet evolved into a year-round community, many of its residents making the commute to New York City.

The hamlet of Mahopac encircles 587-acre (238 ha) Lake Mahopac. It contains three islands, Fairy, Petre, and Canopus, all privately owned. Fairy Island sports multiple homes accessible via a short causeway; Petre Island boasts the Massaro House, a Frank Lloyd Wright-inspired residence, and a Wright-designed bungalo, the A. K. Chahroudi Cottage; Canopus is undeveloped. Boating, fishing and other water sports are permitted on the lake. Slips and support services are provided by two marinas.

The world record brown bullhead (Ameiurus nebulosus), which weighed 7 pounds 6 ounces (3.3 kg), was caught in Lake Mahopac on August 1, 2009 by angler Glenn Collacuro.

Besides Lake Mahopac, other lakes within the Mahopac CDP include Kirk Lake, Lake Casse, Lake Secor, Teakettle Spout Lake, and Long Pond. Lake Mahopac is within the Croton River watershed yet not a part of the New York City water supply system's Croton Watershed. Kirk Lake, a controlled lake in that system, is in both.

According to the United States Census Bureau, the Mahopac CDP has a total area of 6.4 square miles (16.7 km), of which 5.3 square miles (13.7 km) is land and 1.1 square miles (2.9 km), or 17.57%, is water.

While the hamlet of Carmel is the seat of government for Putnam County, Mahopac is the largest population center in the town of Carmel and hosts its Town Hall.

Both Mahopac (ZIP code 10541) and Mahopac Falls (10542) have their own post offices.

Historical population

Census	Pop.	Note	%±
1990	7,755		—
2000	8,478		9.3%
2010	8,369		−1.3%
2020	8,932		6.7%
U.S. Decennial Census

As of the census of 2020, there were 8,932 people and 3,232 households residing in the hamlet. The population density was 1,457.3 inhabitants per square mile (562.7/km) The racial makeup of the CDP was 77.1% White, 1.4% African American, 0.2% Native American, 3.5% Asian, 0% from other races, and 5.4% from two or more races. Hispanic or Latino of any race were 18.4% of the population.

There were 2,943 households, out of which 23.0% had children under the age of 18 living with them, 65.6% were married couples living together, 8.6% had a female householder with no husband present, and 22.7% were non-families. 17.6% of all households were made up of individuals, and 5.6% had someone living alone who was 65 years of age or older. The average household size was 2.74 and the average family size was 3.28.

In the hamlet the population was spread out, with 25.3% under the age of 18, 7.7% from 18 to 24, 30.8% from 25 to 44, 26.4% from 45 to 64, and 9.7% who were 65 years of age or older. The median age was 38 years. For every 100 females, there were 99.7 males. For every 100 females age 18 and over, there were 96.7 males.

The median income for a household in the community was $95,189, and the median income for a family was $91,148. Males had a median income of $52,315 versus $36,419 for females. The per capita income for the CDP was $44,494. The percent of persons below poverty level was 5.0%.

The median value of owner-occupied housing units was $407,900.

The Mahopac Central School District covers the census-designated place.

The district is divided into five schools: three K-5 schools (Lakeview Elementary School, Fulmar Road Elementary School, and Austin Road Elementary School), a middle school for grades 6-8 (Mahopac Middle School) and a high school for grades 9-12 (Mahopac High School).

The name "Mahopac" is believed to be an indigenous word meaning "Great Lake" or "Lake of the Great Serpent". Long-time area residents affirm that the pronunciation "Ma-HO-pac" – consistent with other Native American named local communities such as Mohegan and Mohansic – was used by residents until the hamlet's large population expansion and demographic shift in the 1960s and '70s.

In spite of a 3–2 vote by the Carmel Town Board in favor of the traditional pronunciation, the modern-day first-syllable-inflected "MAY-o-pac" is used by a large number of residents today. Stressing the similar pronunciation of other Native American communities nearby, lifelong Mahopac resident and retired Putnam County legislator and assistant to the Putnam County Executive Regina Morini affirmed, "Mohegan, Mohansic, Mahopac. The accent is on the second syllable".

• New York and Putnam Railroad, which first brought service to Mahopac
• USS Mahopac, any of three namesake U.S. Navy ships: a Civil War era monitor, a post-WWI fleet tug, and a World-War-II-era rescue tugboat

• Mahopac News
• Mahopac Real Estate
• Town of Carmel
• Mahopac Central School District
• Mahopac Public Library
• Mahopac Volunteer Fire Department
• Town of Carmel Police Department
• Greater Mahopac-Carmel Chamber of Commerce