[9] After graduating from Tisch, Willems spent a year traveling around the world drawing a cartoon every day, all of which have been published in the book You Can Never Find a Rickshaw When it Monsoons.

[10] Returning to New York, he started his career as a writer and animator for Sesame Street, where he earned six Emmy Awards for writing during his tenure from 1993 to January 2002.

During this period he also performed stand-up comedy in NYC and recorded essays for BBC Radio along with making a promo for Cartoon Network and animating the opening for a show on Nickelodeon.

Willems later worked as head writer on the first four seasons of Codename: Kids Next Door,[12] created by one of his colleagues from Sheep, Tom Warburton.

[15] Willems' books have been translated into a number of languages, spawned animated shorts that have twice been awarded the Carnegie Medal (Knuffle Bunny, 2007,[16] and Don't Let the Pigeon Drive the Bus, 2010[17]), and been developed into theatrical musical productions.

[22] Two animated versions of his books were awarded Carnegie Medals (Knuffle Bunny: A Cautionary Tale and Don't Let the Pigeon Drive the Bus!

and Are You Ready to Play Outside?, and five Geisel Honors, for We Are in a Book!, I Broke My Trunk!, Let's Go for a Drive!, A Big Guy Took My Ball!, and Waiting Is Not Easy!

[27] Willems won the Daytime Emmy Award for Outstanding Writing For A Children's Series six times for his work on Sesame Street between 1995 and 2001.