[5] It is a population center of Traditionalist Catholicism and the home of Saint Mary's Academy and College.

[8] 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.

Starting in the 1980s, Traditionalist Catholics attending masses offered by Society of Saint Pius X began moving to St. Marys, attracted by the presence of the Society in the community as well as the relative isolation of the city from the modern world.

Since then the number of residents in St. Marys affiliated with the SSPX has grown considerably, to the point where in 2020 that they formed the majority of the city's population.

The growth of the SSPX has been a source of tension with some older residents who are not members of the movement and who do not share the Society's socially conservative values.

[9] According to the United States Census Bureau, the city has a total area of 1.18 square miles (3.06 km2), all land.

[14] The Immaculata was consecrated in a ceremony on May 3, 2023 by Bishop Bernard Fellay, as the final step of a plan written forty-five years prior to the dedication.

[19] According to the 2020 census, the majority of St. Marys' inhabitants are Catholics affiliated with the SSPX; this has created tensions with some older residents who are not followers of the movement and who do not share the SSPX's socially conservative values.

[9][20] The community is served by Kaw Valley USD 321 public 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.