While still following several art studies, Leonard Starr worked on several comic series from 1942. He collaborated on 'The Sub-Mariner', 'Human Torch' and 'Don Winslow', before he started a career in the advertising field. In 1957, he returned to comics with 'On Stage', a series about actress 'Mary Perkins' that appeared in The Chicago Tribune. He drew this series until 1979. In 1979, Starr took over the 'Little Orphan Annie' comic, originally created by Harold Gray.
Leonard Starr also worked for the European market, producing, among others, 'Cannonball Carmody' for Tintin. He wrote the scenarios for 'Kelly Green', illustrated by Stan Drake, that appeared in Pilote and Charlie Mensuel. From the early 1970s, Starr worked for the American television. For example, he contributed to the 'Thundercats' cartoons from the 1980s.