A child model from age two, Larry Simms was discovered by a Hollywood talent scout when he appeared in a 1937 Saturday Evening Post advertisement. The three-year-old, curly haired Simms made his screen debut as the infant son of Jimmy Stewart and Rose Stradner in MGM's The Last Gangster. He was then hired by Columbia to play Baby Dumpling in the 1938 cinemadaptation of Chic Young's comic strip Blondie. Simms remained with the Blondie series until its cessation in 1950, billed onscreen as Baby Dumpling until his character name was formalized as Alexander Bumstead. During this period, he also made a few "outside" appearances in films like Mr. Smith Goes to Washington (1939) and Madame Bovary (1949). Though his career as a child star was a pleasant experience (and, at 750 dollars per week, a lucrative one), Simms wasn't all that interested in acting; the technical end of moviemaking was more fascinating to him. In 1950, he quit show business to join the Navy, then studied aeronautical engineering at California Polytech. Larry Simms was then hired as an engineer at the Jet Propulsion Laboratory in Pasadena, where he remained until his retirement.