William Schallert was born in Los Angeles, California, the son of Edwin Francis Schallert, a longtime drama critic for the "Los Angeles Times", and Elza Emily Schallert (née Baumgarten), a magazine writer and radio host. He began acting while a student at the University of California, Los Angeles (UCLA) but left to become an Army Air Corps fighter pilot in World War II. He returned to UCLA after the war and graduated in 1946. In 1946, he helped found the Circle Theatre with Sydney Chaplin and several fellow students. In 1948, Schallert was directed by Sydney's father, Charlie Chaplin, in a staging of W. Somerset Maugham's "Rain". In 1949, Schallert served as the reciter in a concert performance of Arnold Schoenberg's "Ode to Napoleon" in celebration of the composer's 75th birthday.