Larry Lee Simms worked as a child model from the age of two years and was discovered by a Hollywood talent scout when he appeared in a 1937 Saturday Evening Post advertisement. His first film for Hollywood was The Last Gangster (1937), where he played Edward G. Robinson's young son. Simms got well-known with his appearances as Alexander "Baby Dumpling" Bumstead in the popular Blondie film series starring Penny Singleton. Between 1938 and the end of the series in 1950, Simms appeared as Alexander in 28 films of the Blondie comedies and was a regular cast member. The child actor earned at one time $750 a week. In 1946, Simms joined the cast of the Blondie radio program, portraying Alexander there as he had in movies.

Larry Simms sometimes also appeared in movies out of the Blondie Series, most notably in two films of Frank Capra: He played one of the sons of Governor Hopper (Guy Kibbee) in Mr. Smith Goes to Washington (1939) and Pete Bailey, the oldest son of James Stewart's George Bailey in It's a Wonderful Life (1946). Not very interested in acting, he quit show Business to join the Navy (he appeared in uniform as himself in the Columbia Screen Snaphots short, Hollywood Grows Up), then studied aeronautical engineering at California Polytech. Simms later worked at Jet Propulsion Laboratory (JPL) in Pasadena, California and around the world as an Engineer until his retirement. In his last years, Larry Simms lived in Thailand with his wife.