St. Marys was laid out as a town in 1866. It was named after the St. Mary's Mission. The oldest surviving mission building is the Pottawatomie Indian Pay Station which was built in 1855 and was used by government agents to pay an annuity to the Pottawatomie Indians who relocated to the area from the Great Lakes region. It was added to the National Register of Historic Places in 1972.
At the 2010 census there were 2,627 people, 836 households, and 560 families living in the city. The population density was. There were 900 housing units at an average density of. The racial makeup of the city was 91.1% White, 1.1% African American, 1.4% Native American, 0.5% Asian, 2.3% from other races, and 3.6% from two or more races. Hispanic or Latino of any race were 9.0%. Of the 836 households 37.6% had children under the age of 18 living with them, 54.4% were married couples living together, 9.0% had a female householder with no husband present, 3.6% had a male householder with no wife present, and 33.0% were non-families. 29.5% of households were one person and 14.7% were one person aged 65 or older. The average household size was 3.10 and the average family size was 4.04. The median age was 26.8 years. 38.5% of residents were under the age of 18; 9.6% were between the ages of 18 and 24; 19.8% were from 25 to 44; 18.7% were from 45 to 64; and 13.6% were 65 or older. The gender makeup of the city was 48.0% male and 52.0% female.
At the 2000 census there were 2,198 people, 818 total households, and 525 families living in the city. The population density was 1,987.1 people per square mile. There were 818 housing units at an average density of 739.5 per square mile. The racial makeup of the city was 95.18% White, 0.91% African American, 1.36% Native American, 0.50% Asian, 0.50% from other races, and 1.55% from two or more races. Hispanic or Latino of any race were 4.14%. Of the 818 households 35.5% had children under the age of 18 living with them, 56.8% were married couples living together, 9.2% had a female householder with no husband present, and 31.2% were non-families. 29.7% of households were one person and 12.4% were one person aged 65 or older. The average household size was 2.83 and the average family size was 3.61. The age distribution was 35.4% under the age of 18, 7.6% from 18 to 24, 24.0% from 25 to 44, 17.9% from 45 to 64, and 15.0% 65 or older. The median age was 31 years. For every 100 females, there were 91.3 males. For every 100 females age 18 and over, there were 88.4 males. The median household income was $28,083 and the median family income was $28,063. Males had a median income of $25,595 versus $23,750 for females. The per capita income for the city was $15,536. About 26.2% of families and 31.1% of the population were below the poverty line, including 17.1% of those under age 18 and 27.2% of those age 65 or over.
Education
The community is served by Kaw Valley USD 321public school district, which is located in St. Marys. Besides providing education from pre-K to 12 in St. Marys, it also services students in a four-county radius with four other attendance centers. Its high school is St. Marys High School. A private school, St. Mary's Academy and College includes grades K-12 as well as a two-year college program. The Society of Saint Pius X operates the school.
