The area surrounding modern-day Groton has, for thousands of years, been the territory of various cultures of indigenous peoples. They settled along the rivers, which they used for domestic tasks, fishing and transportation. Historic tribes were the Algonquian-speaking Nipmuc and Nashaway Indians.

The Anglo-American Groton started with the trading post of John Tinker, who conducted business there with the Nashaway at the confluence of Nod Brook and the Nashua River. The Nashaway called the area Petapawag, meaning “swampy land.” As Tinker had, other pioneers followed the Algonquian trails from Massachusetts Bay. They found the region productive for fishing and farming.

The town was officially settled and incorporated in 1655, named for Groton in Suffolk, England. Called The Plantation of Groton, it included all of present-day Groton and Ayer, almost all of Pepperell and Shirley, large parts of Dunstable, Littleton, and Tyngsborough, plus smaller parts of Harvard and Westford in Massachusetts, as well as Nashua and Hollis, New Hampshire.

During King Philip’s War, on March 13, 1676, Native Americans burned all buildings except for four Groton garrisons. Among those killed was John Nutting, a Selectman at Groton. Survivors fled to Concord and other safe havens, but two years later returned to rebuild. Native Americans attacked the town again during the Raid on Groton in 1694 (during King William’s War).

In 1704 during Queen Anne’s War, a French-Abenaki raid captured three children of Thomas Tarbell and his wife, among others, taking them overland about 300 miles to the Mohawk village of Kahnewake south of Montreal, where they would be held for ransom or adopted into the tribe by individual Mohawk families. The trade in captives was a thriving business between the opposing colonies of the English and French. The two Tarbell boys, John and Zachariah, were adopted by Mohawk families and became fully assimilated, later marrying into the tribe, having families, and becoming chiefs. They were among the founders in the 1740s of Akwesasne, after moving up the St. Lawrence River from Kahnewake. The brothers’ older sister Sarah Tarbell was ransomed by a French family, and converted to Catholicism. She joined a Catholic teaching/nursing religious order in Montreal and served with them for the rest of her life. There are Tarbell-named descendants among Mohawk of Kahnewake and Akwesasne in the 21st century.

In 1775, the common in front of the First Parish Church was an assembly area for Minutemen who fought in the Battle of Lexington and Concord.

In the 19th century and early part of the 20th century, Groton was a largely white and Christian town and may have been a sundown town. The town was a center of Ku Klux Klan activities that included anti-Catholic prejudice and prejudice against ethnic minorities. In September of 1924, there was a gathering of Klansmen in the town with approximately fifty automobiles with identity protected professionals and merchants from the area. Many townspeople opposed the Klan, and this active protest resulted in some violence. In October of 1926, a group of 400 Klansmen where having a meeting in a field in the town when they were fired upon by a group of approximately 100 people opposed to the Klan. The police reported that over 100 gunshots were exchanged between the two groups, but no casualties were reported. Nearly 100 years later, in 2020, in recognition of the violence of its past, Groton unanimously approved a measure denouncing racial bigotry and advocating equality. Similarly, in 2017, in the shadows of the environment created by the 2016 presidential election, the town adopted the motto “All Are Welcome”, a statement of its openness and tolerance. 6 stones were engraved with the motto and placed on the major roads entering the town from neighboring communities

According to the United States Census Bureau, Groton has a total area of 33.7 square miles (87.3 km²), of which 32.8 square miles (84.9 km²) is land and 0.9 square miles (2.4 km²) (2.79%) is water. Groton is the largest town in Middlesex County in terms of square mileage. The town is drained by the Nashua River, Squannacook River, and Merrimack River. The center of the town is dominated mainly by Gibbet Hill, with several other large hills throughout the town.

Groton is served by state routes 40, 111, 119 and 225. It borders the towns of Pepperell, Dunstable, Tyngsborough, Westford, Littleton, Ayer, Shirley, and Townsend.

Groton has a hot-summer humid continental climate (Dfa) bordering on Dfb and monthly averages range from 23.8° F in January to 71.8° F in July. The hardiness zone is 5b.

See also: Groton (CDP), Massachusetts

As of the census of 2000, there were 9,547 people, 3,268 households, and 2,568 families residing in the town. The population density was 291.3 people per square mile (112.5/km²). There were 3,393 housing units at an average density of 103.5 per square mile (40.0/km²). The racial makeup of the town was 97.22% White, 0.35% Black or African American, 0.13% Native American, 0.97% Asian, 0.02% Pacific Islander, 0.27% from other races, and 1.04% from two or more races. Hispanic or Latino of any race were 1.14% of the population.

There were 3,268 households, out of which 46.8% had children under the age of 18 living with them, 70.0% were married couples living together, 6.3% had a female householder with no husband present, and 21.4% were non-families. Of all households 17.1% were made up of individuals, and 5.4% had someone living alone who was 65 years of age or older. The average household size was 2.90 and the average family size was 3.31.

The age distribution of the town’s population was 32.6% under the age of 18, 4.2% from 18 to 24, 32.7% from 25 to 44, 23.5% from 45 to 64, and 7.0% who were 65 years of age or older. The median age was 36 years. For every 100 females, there were 98.2 males. For every 100 females age 18 and over, there were 94.5 males.

The median income for a household in the town was $118,041, and the median income for a family was $136,653. Males had a median income of $101,117 versus $60,402 for females. The per capita income for the town was $44,756. About 1.1% of families and 1.8% of the population were below the poverty line, including 1.0% of those under age 18 and none of those age 65 or over.

The town is governed by an open Town Meeting and administered by an elected Board of Selectmen and appointed Town Manager.