Lively's first film was as a baby in the 1983 film Brainstorm. After graduating high school, Lively moved to New York City to study photography, his first passion. He studied photography at Parsons The New School for Design in New York City. At one time, he was an Abercrombie and Fitch model.

He guest starred on an episode of Full House at age 13, where he played Stephanie Tanner's first boyfriend. He had a role in American Pie (1999). He followed this up on the television series So Weird, on which he played Carey Bell from 2000 to 2001. He learned to play the guitar for his character on So Weird. Lively then became a cast member of the short-lived 2003 Norm Macdonald series A Minute With Stan Hooper. In 2004 he acted in Speak with Kristen Stewart. In 2005, Lively was offered two different television roles - on 24 and The L Word. He took the role on The L Word as documentary filmmaker Mark Wayland. He chose a recurring role on The L Word over a regular role on 24 because he felt the subject matter was more important. He was part of the cast of the WB series Modern Men from March until May 2006. He was the main character in The Butterfly Effect 2 in 2006.

Lively appeared as Pink's on-screen boyfriend in her music video "Please Don't Leave Me" in 2009, with stuntman Justin Sundquist serving as his stunt double. In 2012, he guest starred in the Lifetime series The Client List.