After moving to the United States in 1922, Reiner served as music director of several major orchestras, including Cincinnati Symphony Orchestra, Pittsburgh Symphony Orchestra and the Metropolitan Opera. He committed to introducing to American audiences the talents from Europe at the time, including Hindemith, Bartók, Stravinsky and Respighi, and he championed American composers, such as Gershwin and Copland. Reiner also taught conducting to students at the Curtis Institute of Music, leaving an indelible mark on Leonard Bernstein, Lukas Foss and Max Goberman.
Reiner was a conductor who was admired by fellow musicians for the breadth and depth of his musical knowledge. He had built a wide repertoire over his career, and music of different styles were performed with incredible nuance and understanding. Reiner's own conducting style was known for being economical, and some claim that the movement of his baton tip to denote every beat only covered the area of a postage stamp.
The contribution Reiner made to the music world - building orchestras, promoting contemporary composers, teaching of conducting - was, to say the least, very significant. Yet, he was an introspective, private man who stayed mostly out of the public eye when not in the concert hall or opera house. As Virgil Thomson states, Reiner was "as calculable as the stars and about as distant."