From Muppet Wiki

John Hoffman, also credited as John Robert Hoffman, is an actor, writer, and director who was a regular on the Disney Channel series Adventures in Wonderland as the Mad Hatter. In 1993, Hoffman appeared in character in the Inaugural Celebration for Children special, along with Kermit the Frog. In 2003, he penned the screenplay and directed the Jim Henson Pictures film Good Boy!