The indigenous occupation of Braintree precedes the arrival of English settlers in 1625. Subsequent to their arrival, the town was colonized in 1635, and ultimately incorporated in 1640. The town is named after the English town of Braintree. In addition to its present boundaries, it comprised land that was later split off into the separate municipalities of Quincy (incorporated in 1792), Randolph (1793), and Holbrook (1872). Braintree was part of Suffolk County until the formation of Norfolk County in 1793.

The town of Braintree is the birthplace of several prominent figures in American history: Abigail Adams, presidents John Adams and John Quincy Adams, statesman John Hancock, and General Sylvanus Thayer, an early superintendent of the United States Military Academy located at West Point, New York.

In 1920, Braintree was the site of the murders that led to the trial of Sacco and Vanzetti. During that same decade, the town’s population grew by over 50%.

In January 2008, Braintree converted from a representative town meeting form of government to a mayor-council government.

Braintree shares borders with Quincy to the north, Randolph to the west (separated by the Cochato River), Holbrook to the south, and Weymouth to the east.

According to the United States Census Bureau, the town has a total area of 14.5 square miles (37.6 km²), of which 13.9 square miles (36.0 km²) is land and 0.6 sq mi (1.6 km²) is water. The total area is 4.34% water

Park and recreation locations in Braintree include Pond Meadow Park, Sunset Lake, and Blue Hills Reservation.

As of the census of 2000, there were 33,828 people, 12,652 households, and 8,907 families residing in the town. The population density was 2,434.4 people per square mile (939.6/km²). There were 12,973 housing units at an average density of 933.6 per square mile (360.4/km²). The racial makeup of the town was 93.96% White, 1.18% Black or African American, 0.11% Native American, 3.14% Asian, 0.03% Pacific Islander, 0.64% from other races, and 0.95% from two or more races. Hispanic or Latino people of any race were 1.16% of the population. More than 46% of town residents had Irish ancestry. As of 2014 Braintree had the 2nd highest concentration of Irish Americans in the entire country, slightly behind Scituate, Massachusetts.

There were 12,652 households, out of which 29.6% had children under the age of 18 living with them, 55.4% were married couples living together, 11.7% had a female householder with no husband present, and 29.6% were non-families. 24.4% of all households were made up of individuals, and 11.9% had someone living alone who was 65 years of age or older. The average household size was 2.61 and the average family size was 3.16.

In the town the population was spread out, with 22.5% under the age of 18, 6.5% from 18 to 24, 28.9% from 25 to 44, 24.0% from 45 to 64, and 18.1% who were 65 years of age or older. The median age was 40 years. For every 100 females, there were 89.1 males. For every 100 females age 18 and over, there were 84.4 males.

The median income for a household in the town was $85,590, and the median income for a family was $90,590 as of a 2007 estimate). Males had a median income of $89,607 versus $36,034 for females. The per capita income for the town was $28,683. About 2.1% of families and 3.8% of the population were below the poverty line, including 4.6% of those under age 18 and 3.3% of those age 65 or over.