Methuen is located at 42°43′48″N 71°10′46″W / 42.73°N 71.17944°W / 42.73; -71.17944 (42.730040, -71.179352). According to the United States Census Bureau, the city has a total area of 23.1 square miles (59.8 km²), of which 22.4 square miles (58.0 km²) are land and 0.7 square miles (1.8 km²) (2.95%) are water. The town lies along the north banks of the Merrimack River, and is also drained by the Spickett River, as well as many brooks and streams. There are several ponds dotting the area as well, and the town is home to a town forest, a bird sanctuary, and a small state park (Tenney State Park). Pine Island, near the southern end of town in the Merrimack River, is also part of the town's land.

Methuen lies along the western edge of Essex County, just south of Rockingham County, New Hampshire. The irregularly-shaped town is bordered by Haverhill to the northeast, North Andover to the east, Lawrence and Andover to the south, Dracut to the west, Pelham, New Hampshire to the northwest, and Salem, New Hampshire to the north. Methuen is located thirty miles north-northwest of Boston, and twenty-five miles south-southeast of Manchester, New Hampshire.

Methuen lies at the northern end of Interstate 93 in Massachuestts, with three exits providing access. A portion of Interstate 495 also crosses through the eastern side of town from Lawrence to Haverhill. Between the two, Massachusetts Route 213, the "Loop Connector," provides highway access between the two, lying entirely within town and having five exits of its own. The town is also crossed by Route 28, Route 110, and Route 113, the latter two meeting at a rotary at I-93 Exit 46, one of the more congested intersections along the I-93 corridor. I-93 also provides the town's only bridge across the Merrimack; there are several crossings in Lawrence, and several in neighboring Haverhill, but none for seven miles upstream from I-93.

Methuen is served by the Merrimack Valley Regional Transit Authority's bus service; there is no other mass transportation source within town. The nearest rail station is in South Lawrence, which is part of the Haverhill/Reading Line of the MBTA Commuter Rail, providing service into Boston's North Station. Small plane service can be found at Lawrence Municipal Airport, with the nearest national service being at Manchester-Boston Regional Airport, and the nearest international service being at Logan International Airport.

As of the census of 2000, there were 43,789 people, 16,532 households, and 11,539 families residing in the city. The population density was 1,954.7 persons per square mile (754.8/km²). There were 16,885 housing units, at an average density of 753.7 per square mile (291.0/km²). The racial makeup of the city was 89.35% White, 1.35% African American, 0.22% Native American, 2.38% Asian, 0.01% Pacific Islander, 4.87% from other races, and 1.82% from two or more races. Hispanics and Latinos, of any race, were 9.64% of the population.

There were 16,532 households, of which 33.1% had children under the age of 18 living with them, 53.3% were married couples living together, 12.2% had a female householder with no husband present, and 30.2% were non-families. 25.3% of all households were made up of individuals and 11.6% had someone living alone who was 65 years of age or older. The average household size was 2.62 and the average family size was 3.17.

In the city the population was spread out with 24.7% under the age of 18, 7.3% from 18 to 24, 31.0% from 25 to 44, 21.6% from 45 to 64, and 15.3% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 91.9 males. For every 100 females age 18 and over, there were 87.0 males.

The median income for a household in the city was $49,627, and the median income for a family was $59,831. Males had a median income of $41,693 versus $31,864 for females. The per capita income for the city was $22,305. About 5.8% of families and 7.4% of the population were below the poverty line, including 9.7% of those under age 18 and 7.7% of those age 65 or over.

Historically, Methuen had a town meeting-selectmen form of government and was known as the Town of Methuen until it adopted a charter replacing its traditional town meeting and selectmen with a council and manager. Even with a form of government that had historically and legally been exclusive to cities, the community, in a gesture of traditionalism, retained the name Town of Methuen in its charter. However, because Massachusetts cities have self-governing powers not available to towns, it became known for legal purposes as "The City Known as the Town of Methuen". A subsequent charter, which adopted a strong mayor form of government, officially changed the community name to the "City of Methuen".

Methuen's city government consists of a Mayor, three Councilors-at-Large, two East District Councilors, two Central District Councilors, two West District Councilors, and six School Committee members.