Levant first went to Hollywood in 1928, composing film scores and songs (frequently with lyricists Sidney Clare, Dorothy Fields and William Kernell) and, after a bit part in the 1929 "The Dance of Life", was featured in best-chum-of-the-star roles from the 40s on. His first major role was as comic foil for Bing Crosby and Mary Martin in "Rhythm on the River" (1940). In the reverent Gershwin biopic, "Rhapsody in Blue" (1945) he played himself (a role for which Levant insisted he was "horribly miscast"); in "An American in Paris" (1951) he was a semi-autobiographical bohemian pianist; and in "The Band Wagon" (1953), he portrayed an Adolph-Green-like Broadway songwriter.
A popular panelist on the radio quiz show, "Information Please" in the 1940s, Levant segued into the role of controversial guest on late night TV in the 50s. Outrageously nasty and curmudgeonly, he paraded his neuroses and illnesses, leavened by his shockingly frank self-awareness, and skewered the famous and near famous with his bitchy repartee: of Zsa Zsa Gabor he said "the only person who ever left the Iron Curtain wearing it" and of hostess Elsa Maxwell: "I once took her to a masquerade party...at the stroke of midnight, I ripped off her mask and discovered I had beheaded her!" About his own TV talk show he commented: "Little did I know when I talked about the lunatic fringe that one day I would be its leader". The titles of his three memoirs relieve the depths of his self-derogatory wit, "A Smattering of Ignorance" (1942), "Memoirs of an Amnesiac" (1965) and "The Unimportance of Being Oscar" (1968).