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. After Phineas and Ferb, Marsh and Povenmire created and produced Milo Murphy's Law, which premiered in 2016. Together they created a second Phineas and Ferb film in 2020, called Candace Against the Universe.
