He worked as an advertising artist for a short time before creating one of his most famous comic strip, On Stage (which was later renamed, Mary Perkins, On Stage) for the Chicago-Tribune-New York News Syndicate. For this strip he received the National Cartoonists Society's Story Comic Strip Award in 1960 and 1963, and the Reuben Award in 1965.
He then ventured into writing television scripts and it was at this juncture in his career when he was hired by Jules Bass to develop thier new animated series, ThunderCats . Starr would turn out to be one of the most influential people in developing the series. From 1985 to 1987, Starr served as the head writer and developer on ThunderCats. He began by writing a "Bible" of the show which contained his ideas for the basic premise, history and characters of the show. He is responsible for creating most of the ThunderCats characters, the Mutants and Mumm-Ra as well as other minor characters like Berbils , Bolkins and Wolos among others.
Starr also contributed artistically to developing the show by drawing the map of Third Earth as well as designing the Sword of Omens and the concept of "Sight Beyond Sight". Starr wrote a number of episodes of the show and is in fact responsible for more scripted episodes than any other writer. His most notable episodes include the pilot, "Exodus" as well as the three 5-parters. Apart from writing scripts he also helped to rewrite many of the scripts penned by other writers.
Episodes written by Leonard Starr
- The Unholy Alliance
- The Slaves of Castle Plun-darr
- The Tower of Traps
- The Ghost Warrior
- The Spaceship Beneath the Sands
- The Crystal Queen
- Lion-O's Anointment First Day: Trial of Strength
- Lion-O's Anointment Second Day: The Trial of Speed
- Lion-O's Anointment Third Day: Trial of Cunning
- Lion-O's Anointment Fourth Day: The Trial of Mind Power
- Lion-O's Anointment Final Day: The Trial of Evil
- ThunderCats - HO! Part 1
- ThunderCats - HO! Part 2
- ThunderCats - HO! Part 3
- ThunderCats - HO! Part 4
- ThunderCats - HO! Part 5
- Mumm-Ra Lives!, Part 1
- Mumm-Ra Lives!, Part 2
- Mumm-Ra Lives!, Part 3
- Mumm-Ra Lives!, Part 4
- Mumm-Ra Lives!, Part 5
- In the original outline prepared by Starr, the names of the characters and objects were slightly different than what they finally became to be known as. Jaga was called Jagu-R, Lion-O was Lion-L, Panthro was Panth-R, Tygra was Tige-R, Cheetara was Cheet-A and Wilykat was Wiley-KT (there was no Wilykit). Also the Eye of Thundera was called Eye of Xanth.