Jeffrey K Marsh (born on December 9, 1960), or better known as Jeff "Swampy" Marsh is an American animation director and voice actor who began working on The Simpsons around 1990, and served as a storyboard artist and layout artist for over six seasons. Simpsons was Swampy's start in the field of animation. While working on The Simpsons, he met his partner and friend Dan Povenmire, and the two would work together down the road.
He is best known as the co-creator of the Disney animated series', Phineas and Ferb and Milo Murphy's Law which he created alongside Povenmire. In addition to being the co-creator and part of the creative team for both series he is also a voice actor. He voices the character Major Monogram on Phineas and Ferb, and voices Cavendish on Milo Murphy's Law. He also developed the Amazon Prime exclusive series Pete the Cat.
In addition to working on The Simpsons and creating his own series, he is also known for his work on the Nickelodeon series Rocko's Modern Life which was the first animated series that he and Povenmire worked on the creative team together. During the long time it took for Phineas and Ferb to finally get picked up, he moved to England after Rocko's Modern Life ended where he worked on British cartoons for a while until he moved back to the United States when Povenmire informed him that Disney had agreed to pick up Phineas and Ferb as an animated series.