Cassell began his film career in 1942, (Another source says, "He ... realized a childhood ambition to become an actor with a role in Labor Pains in 1937.") initially appearing in small, uncredited roles. Mickey Rooney, with whom Cassell appeared in the film noir Quicksand (1950), is credited with suggesting the change of name to Wally Cassell. Rooney is also credited with helping Cassell gain a screen test and a contract with Metro-Goldwyn-Mayer.
His films include The Thin Man Goes Home (1945), The Story of G.I. Joe (1945), The Clock (1945), The Postman Always Rings Twice (1946), Guilty (1947), Loves of Carmen (1948), Saigon (1948), Sands of Iwo Jima (1949), White Heat (1949),Quicksand (1950), City That Never Sleeps (1953), Island in the Sky (1953), Law and Order (1953), Princess of the Nile (1954), Until They Sail (1957), and I, MadMan (1989).
Cassell later appeared in two syndicated programs starring Jim Davis: Stories of the Century, in the role of gunman Luke Short, and Rescue 8, as Johnny French in "One More Step." Cassell also guest-starred in several television series, including The Loretta Young Show (1955), Gunsmoke (1956), the 1959 premier episode of The Untouchables ("The Empty Chair"), Rawhide (1960), and The Beverly Hillbillies (1963).
Cassell retired from acting in 1964 and became a successful businessman.
Cassell died at the age of 103 at his home in Palm Desert, California on April 2, 2015. He was the last surviving actor in most of his films due to his longevity.
