The town is on the east bank of the estuary of the Mystic River, and was long a leading seaport of the area. The story is told at the Mystic Seaport, the world's largest maritime museum, which has preserved both a number of sailing ships (most notably the whaler Charles W. Morgan) and the seaport buildings ashore.
The Mystic Aquarium is notable for its beluga whales.
According to the United States Census Bureau, the town has a total area of 9.8 km² (3.8 mi²). 8.7 km² (3.3 mi²) of it is land and 1.1 km² (0.4 mi²) of it is water. The total area is 11.61% water.
As of the census of 2000, there are 4,001 people, 1,797 households, and 995 families residing in the town. The population density is 461.1/km² (1,192.7/mi²). There are 1,988 housing units at an average density of 229.1/km² (592.6/mi²). The racial makeup of the town is 95.78% White, 0.80% African American, 0.40% Native American, 1.35% Asian, 0.02% Pacific Islander, 0.35% from other races, and 1.30% from two or more races. 1.35% of the population are Hispanic or Latino of any race.
There are 1,797 households out of which 20.3% have children under the age of 18 living with them, 45.6% are married couples living together, 7.0% have a female householder with no husband present, and 44.6% are non-families. 35.3% of all households are made up of individuals and 9.1% have someone living alone who is 65 years of age or older. The average household size is 2.10 and the average family size is 2.76.
In the town the population is spread out with 16.7% under the age of 18, 5.4% from 18 to 24, 31.1% from 25 to 44, 27.4% from 45 to 64, and 19.4% who are 65 years of age or older. The median age is 43 years. For every 100 females there are 92.7 males. For every 100 females age 18 and over, there are 90.1 males.
The median income for a household in the town is $62,236, and the median income for a family is $70,625. Males have a median income of $50,036 versus $32,400 for females. The per capita income for the town is $33,376. 3.1% of the population and 1.6% of families are below the poverty line. Out of the total people living in poverty, 1.9% are under the age of 18 and 5.9% are 65 or older.