He, along with the other Mud Dogs, had already been arrested for an unspecified crime, when Raph was arrested after he was mistaken for Heinous Green. The Mud Dogs already respected Heinous Green by reputation, so they decided to make him their new leader. Loathsome Leonard was disappointed by this.
Raph went along with the mistaken identity and pretended to be Heinous Green, all the while trying to get the Mud Dogs arrested by the real Hidden City Police. But not only did each of Raph's plans backfire and actually help the Mud Dogs, but they came to see "Heinous Green" as a brilliant criminal genius. But Raph's cover was blown when the Mud Dogs went to the give a cut of their stolen loot to Big Mama.
Leonard along with the other Mud Dogs was angry at having been lied to, and they tried to attack Raph, but missed their target and hit Big Mama instead.
It is unknown what happened to Dastardly Leonard, but Raph reported the Mud Dogs' activities to the police before Raph himself was again detained.
- Loathsome Leonard is an obvious parody of Leonardo's counterparts in both the 2012 TV series and in Rise of the TMNT itself.
- His voice actor, Seth Green, voiced Leonardo for the last three seasons of the 2012 TV series.