Born Paul Sternberg in New York City, Stewart graduated from Columbia University and made his Broadway theatre debut in the play Two Secondsin 1931. He soon turned up as a radio regular, perhaps his most familiar role being Jane Ace's shiftless, scheming brother Johnny in Easy Aces. A few years later he met Orson Welles, who invited him to join the Mercury Theatre, where he participated in the notorious 1938 radio broadcast of The War of the Worlds. He was a founding member of AFTRA.
Sternberg's many screen credits include Citizen Kane, Twelve O'Clock High, Champion, Kiss Me Deadly, The Bad and the Beautiful, In Cold Blood,The Day of the Locust, S.O.B., and W.C. Fields and Me, in which he portrayed Florenz Ziegfeld. On television, he appeared in Playhouse 90, Alfred Hitchcock Presents, Alcoa Theatre, Dr. Kildare, Wagon Train, Gunsmoke, It Takes a Thief, Mannix, The Name of the Game, Columbo: Double Shock, McMillan & Wife, Mission Impossible, Ellery Queen, The Rockford Files, Lou Grant, and Remington Steele, among many others. He also made two guest appearances on Perry Mason during its final two seasons in 1964 and 1966. He also directed some television episodes, among them "Little Girl Lost", from the Twilight Zone.
Stewart was married to big band singer/actress Peg La Centra from 1939 until his death from a heart attack in Los Angeles at age 77.
In the 1999 television movie RKO 281, he was portrayed by Adrian Schiller.