Leondis started his career as a story Artist working on films such as The Prince of Egypt and The Lion King II: Simba's Pride. He later joined DisneyToon Studios, where he co-wrote the script for Kronk's New Groove. Leondis made his directorial debut on Lilo & Stitch 2: Stitch Has a Glitch in 2005. In 2008, he directed his first theatrical animated film, Igor, for Exodus Film Group.
Since 2009, he was directing at DreamWorks Animation an animated feature film about ghosts, titled B.O.O.: Bureau of Otherworldly Operations. The film, which was based on his original idea, was scheduled to be released in 2015, but by late 2014, it was pulled from DWA's schedule and went back into restructuring.
The film was released on July 28, 2017. Though the film was a box-office success, it was panned by critics. He received two Golden Raspberry Award nominations for Worst Director and Worst Screenplay, the latter he shared with Eric Siegel and Mike White, making him the first animation Director to win in those categories.
