While in England, Marsh worked on several animated programs, including "Postman Pat" and "Bounty Hamster," and worked for BKN New Media Ltd. to produce several feature films. After six years living in England, Marsh was asked by his longtime partner Dan Povenmire to help produce "Phineas and Ferb" in 2007, a concept the two had while working together on "Rocko's Modern Life." Marsh accepted and moved back to the United States; the series has since garnered Marsh two Emmy Awards nominations for songwriting. According to an original pitch extra of "Phineas and Ferb", he prefers having "Swampy" in his name, because it made his name 15 letters long and longer than most other people's names. As of 2015, he and Povenmire are currently producing a new series for Disney XD titled "Milo Murphy's Law", which premiered on October 3, 2016. Marsh has also developed an animated TV series for Amazon Prime based on the Pete the Cat children's book series, with the first season premiering in September 2018. His recent work includes "", which was released on Disney+ on August 28, 2020.