Marengo is a town in and the county seat of Iowa County, Iowa, United States. It has served as the county seat since August 1845, even though it was not incorporated until July 1859. The population was 2,528 in the 2010 census, a decline from 2,535 in 2000.
Marengo was laid out in 1845 and platted in 1847. The city's name commemorates the Battle of Marengo, where Napoleon defeated the Austrian army.
According to the United States Census Bureau, the city has a total area of 2.14 square miles (5.54 km2), of which 2.08 square miles (5.39 km2) is land and 0.06 square miles (0.16 km2) is water.
The Iowa River passes north of town.
As of the census of 2010, there were 2,528 people, 1,059 households, and 648 families living in the city. The population density was 1,215.4 inhabitants per square mile (469.3/km2). There were 1,154 housing units at an average density of 554.8 per square mile (214.2/km2). The racial makeup of the city was 97.3% White, 0.6% African American, 0.6% Native American, 0.4% Asian, 0.4% from other races, and 0.7% from two or more races. Hispanic or Latino of any race were 2.8% of the population.
There were 1,059 households of which 30.5% had children under the age of 18 living with them, 45.2% were married couples living together, 10.3% had a female householder with no husband present, 5.7% had a male householder with no wife present, and 38.8% were non-families.