St. Louis Country Day School's campus was in a bucolic setting reached by electric streetcar, far removed from the noise and grit of the city.

Mary Institute moved to its Ladue campus in 1931 and became independent of Washington University in 1949.

The Mary Institute building contains a three-figure bronze sculpture by Cyrus Dallin: Alma Mater, honoring schoolmaster Edmund Hamilton Sears and donated by Eliza Northrop McMillan.

[3] By the 1950s, the tranquility of the Country Day campus was disrupted by the growth of the adjacent Lambert–Saint Louis International Airport.

In the 1970s, Mary Institute and Country Day began expanding their long-existing connections, including theatrical cooperation, into academic coordination.

St. Louis Country Day headmaster John Johnson, who coordinated the merger, became head of the combined schools.

In 2013, MICDS opened a STEM building on the Upper School campus that contained classrooms, an auditorium, a hearth room, and student commons.

[7] MICDS also has a cross-state rivalry with The Pembroke Hill School in Kansas City.

Pro Football Hall of Fame inductee Marv Levy began his coaching career here in 1951, staying for two years.