Jason Segel was born in Los Angeles, California and raised in Pacific Palisades, California. He attended a Catholic school, though he is Jewish. As a boy, he graduated from Harvard-Westlake School. He was hoping to become an actor during his school days and acted in local theater productions at Palisades Playhouse.

He is known for his role as "Freak" Nick Andopolis on the short-lived NBC comedy-drama series Freaks and Geeks, about a group of suburban Detroit high school students circa 1980. He also personally wrote a song for his character, Nick, to sing to the lead female character, Lindsay (Linda Cardellini, in an episode of Freaks and Geeks. She and Segel dated for a few years following the show's cancellation.

Segel had recurring roles on CSI: Crime Scene Investigation as Neil Jansen and on Undeclared as Eric. When Seth Rogenplanned to star in Superbad, Jason was attached to play Evan, as revealed by the DVD. He currently plays Marshall on the CBS sitcom How I Met Your Mother, which debuted on September 19, 2005. Feature film appearances include Slackers, SLC Punk!, The Good Humor Man, and Dead Man on Campus. In 2007, he appeared in Knocked Up, starring Katherine Heigland Seth Rogenand directed by Freaks and Geeks producer Judd Apatow. Segel starred in the lead role of 2008's Forgetting Sarah Marshall, a film he wrote and Apatow produced, with Shauna Robertson, for Universal Pictures. Next, he will begin filming I Love You, Man for Dreamworks.

Segel, along with Nicholas Stoller, has recently been enlisted by Disney to write and direct the next Muppets film.