The South Dakota Department of The American Legion was organized simultaneously with the national organization. South Dakota Adjutant General William A. Morris named Theodore R. Johnson of Sioux Falls to select a committee of representative ex-service men to attend the first caucus in St Louis, Missouri, May 8-10, 1919.

After the first national caucus, a temporary South Dakota organization was formed at Vermillion on May 15, 1919 with Theodore R. Johnson as chairman. The first state convention was held in Sioux Falls on July 14 and 15, 1919. Myron L. Shade of Mitchell was elected the first Department Commander. Forty-eight posts had been organized by convention time.

The South Dakota Department now consists of about 260 posts with about 26,000 members.

One of the American Legion programs with South Dakota roots is American Legion baseball, now a nationwide program. It was conceived at the state convention at Milbank in 1925. Frank Sieh of Aberdeen originated the idea. A granite monument at Milbank commemorates the formation of Legion Baseball there.

The South Dakota American Legion Department  Headquarters is located in the Codington County courthouse in Watertown. The South Dakota Legion contributed to the construction cost of that courthouse and thus was granted rent-free space in the courthouse for the life of the building.