Born in Brooklyn, New York to voice actor Allen Swift, Stadlen studied acting with Sanford Meisner and Stella Adler. He made his Broadway debut as Groucho Marx in the musical comedy "Minnie's Boys" in 1970. Other noted broadway roles include Senex in "A Funny Thing Happened On The Way To The Forum", Banjo in a revival of "The Man Who Came to Dinner", Milt in "Laughter on the 23rd Floor," and Dr. Pangloss in the 1973 production of "Candide." He has been nominated for 2 Tony awards over the years.