St. Mary's Institute of O'Fallon

St. Mary's Institute is a convent and school complex in O'Fallon with a three-story Gothic Revival motherhouse featuring interconnected wings. The building contains two chapels, classrooms, a dining hall, kitchen, gymnasium, and infirmary spaces.

The original structure was designed and built in 1874 by architect Henry Kister. Subsequent additions and expansions were made over the following decades, concluding around 1997.

The complex serves the Congregation of the Sisters of the Adoration of the Most Precious Blood and maintains their religious educational mission. The sisters have shaped spiritual and educational life here for many generations through their work.

The property spans approximately 43 acres with eleven contributing structures listed on the National Register. Part of the complex now serves public purposes, including municipal government offices.

The grounds include a cemetery that is part of the historic district, holding graves of multiple generations of sisters. The architectural blend of Gothic and Colonial Revival styles across the property reflects different building phases.