McKenzie was born in Austin, Texas to Pieter Meade Schenkkan and Mary Frances Victory, a prize-winning poet. His uncle is the Pulitzer Prize-winning playwright Robert Schenkkan. He played football at Austin High School before attending the University of Virginia, where he majored in Foreign Affairs and Economics. He got into acting during his first few years at the university, and moved to Los Angeles in late 2001 to look for acting opportunities.

McKenzie appeared in minor television roles before being cast as Ryan Atwood, a troubled teen thrust into a new life in Newport Beach, on The O.C., which was a ratings success during the first few seasons, but was later canceled due to low ratings. The O.C. ended on February 22nd, 2007.

During the first season of The O.C rumors began to spread about McKenzie dating co-star Mischa Barton. However they never announced they were a couple.

McKenzie has volunteered as an actor with the Young Storytellers Program.

He is a fan of Major League Baseball, specifically a fan of the Boston Red Sox, as he stated on the August 12, 2006 episode of This Week In Baseball.

On a promotional tour to the UK in 2005 he appeared on MTV and was asked if he was going to see any bands while he was over there, he responded that he really wanted to see Oasis but they were sold out.

As of 2005, he became the third star of The O.C. to appear on Punk'd. He set up his co-star Adam Brody in 2004, and he appeared again on Punk'd in 2005, this time getting punk'd by Kutcher's crew members.

During the 2004 Democratic National Convention and in support of presidential candidate Senator John F. Kerry, McKenzie gave a speech appealing to young Americans to go out and vote.