Marshall is best known for its cross-section of 19th- and early 20th-Century architecture. It has been referred to by the keeper of the National Register of Historic Places as a "virtual textbook of 19th-Century American architecture." It is home to the one of the nation's largest National Historic Landmark Districts. There are over 850 buildings included in the Landmark.
As of the census of 2000, there were 7,459 people, 3,111 households, and 1,935 families residing in the city. The population density was 1,260.7 per square mile (486.5/km²). There were 3,353 housing units at an average density of 566.7/sq mi (218.7/km²). The racial makeup of the city was 95.91% White, 0.32% African American, 0.43% Native American, 0.59% Asian, 0.99% from other races, and 1.76% from two or more races. Hispanic or Latino of any race were 3.16% of the population.
There were 3,111 households out of which 30.9% had children under the age of 18 living with them, 48.5% were married couples living together, 10.0% had a female householder with no husband present, and 37.8% were non-families. 32.9% of all households were made up of individuals and 15.0% had someone living alone who was 65 years of age or older. The average household size was 2.33 and the average family size was 2.98.
In the city the population was spread out with 25.0% under the age of 18, 7.3% from 18 to 24, 28.2% from 25 to 44, 21.2% from 45 to 64, and 18.3% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 86.0 males. For every 100 females age 18 and over, there were 81.2 males.
The median income for a household in the city was $41,171, and the median income for a family was $53,317. Males had a median income of $41,446 versus $30,398 for females. The per capita income for the city was $22,101. About 2.6% of families and 5.0% of the population were below the poverty line, including 3.2% of those under age 18 and 3.9% of those age 65 or over.