Designed by Rapp & Rapp, the 90,000-square-foot (8,400 m²) theater opened on October 30, 1921 as the Mainstreet Missouri.
The 3,200 seat theater was a popular vaudeville and movie house, and the only theater in Kansas City designed by Chicago firm Rapp and Rapp.
The interior was designed in French Baroque style, and the exterior is a blend of neoclassical and French Second Empire.
The lobby is topped by a dome encircled by circular windows.
The Mainstreet Theater was the largest theater in Kansas City until the Midland Theatre opened in 1927.

The Mainstreet was the first theater in Kansas City to have a nursery for children whose parents were attending a show.
Located in the basement and staffed by a trained nurse, it offered toys and games for older children and cribs for babies.

A tunnel connected the theater's lower level to the nearby President Hotel at 14th and Baltimore.
Built to allow actors to enter the theater from their dressing rooms, it became infamous as a passage for bootleggers to escape police during Prohibition.
The supposed tunnel is not shown on the Sanborn fire insurance map for 1939 (The Sanborn Map Co., New York, Kansas City 1939-1940 vol.
1, 1939, Sheet 34).
 
The theater also had space in the basement and sub-basement to keep animals for vaudeville shows.
The space included an elephant cage, a pool for seals, and an elevator large and powerful enough to haul elephants to the stage.
Noted performers such as Cab Calloway, Charlie Chaplin, Harry Lauder, the Marx Brothers, and Olsen and Johnson headlined at the vaudeville house.
In the early 1920s, at the height of the theater's popularity, attendance averaged over 4,000 daily.

The Mainstreet Theater's name changed to the RKO Missouri Theater in April 1941.
The RKO Missouri ran Cinerama three strip film.
Designed by Rapp & Rapp, the 90,000-square-foot (8,400 m²) theater opened on October 30, 1921 as the Mainstreet Missouri. The 3,200 seat theater was a popular vaudeville and movie house, and the only theater in Kansas City designed by Chicago firm Rapp and Rapp. The interior was designed in French Baroque style, and the exterior is a blend of neoclassical and French Second Empire. The lobby is topped by a dome encircled by circular windows. The Mainstreet Theater was the largest theater in Kansas City until the Midland Theatre opened in 1927.
The Mainstreet was the first theater in Kansas City to have a nursery for children whose parents were attending a show. Located in the basement and staffed by a trained nurse, it offered toys and games for older children and cribs for babies. 
A tunnel connected the theater's lower level to the nearby President Hotel at 14th and Baltimore. Built to allow actors to enter the theater from their dressing rooms, it became infamous as a passage for bootleggers to escape police during Prohibition. The supposed tunnel is not shown on the Sanborn fire insurance map for 1939 (The Sanborn Map Co., New York, Kansas City 1939-1940 vol. 1, 1939, Sheet 34).  
The theater also had space in the basement and sub-basement to keep animals for vaudeville shows. The space included an elephant cage, a pool for seals, and an elevator large and powerful enough to haul elephants to the stage. Noted performers such as Cab Calloway, Charlie Chaplin, Harry Lauder, the Marx Brothers, and Olsen and Johnson headlined at the vaudeville house. In the early 1920s, at the height of the theater's popularity, attendance averaged over 4,000 daily.
The Mainstreet Theater's name changed to the RKO Missouri Theater in April 1941. The RKO Missouri ran Cinerama three strip film.
Mainstreet Theater