He was born in Budapest in Hungary and studied music there at the Franz Liszt Academy. After early engagements at opera houses in Budapest and Dresden he moved to the United States of America in 1922 (becoming an American citizen in 1928) to take the post of Principal Conductor of the Cincinnati Symphony Orchestra. He held the post until 1931, then began to teach at the Curtis Institute in Philadelphia, Pennsylvania, his pupils there including Leonard Bernstein and Lukas Foss. He conducted the Pittsburgh Symphony Orchestra from 1938 to 1948, but it was as conductor of the Chicago Symphony Orchestra (1953-63) that he found greatest fame.
Reiner died in New York City.