In 2006, he collaborated with DJ Mark Ronson, to cover the Radiohead song, "Just". The cover appears on the compilation album Exit Music: Songs with Radio Heads, as well as the Mark Ronson album, Version. He appeared with Ronson at the 2007 Glastonbury Festival where he climbed up on to the lighting rig of the John Peel stage during the performance of "Just", and also sang "California". In 2010, he collaborated once again with Ronson on the album "Record Collection", on which Greenwald shares writing credits on six out of the eleven tracks. He became part of Ronson's band, The Business Intl. for this record. He was also a member of Los Angeles band blackblack, until 2007. At live shows, he was known for 1980s styled dance moves, which were inspired from watching David Byrne of the Talking Heads.

Greenwald was born in Los Angeles, California, to a family of Jewish descent. In 1994, Greenwald formed Phantom Planet in his teens with four friends – Guitarist Darren Robinson, Bassist Sam Farrar, Guitarist Jacques Brautbar, and Drummer Jason Schwartzman. The band is best known for producing the song "California", which was used as the theme song of the teen drama The O.C., which lasted four seasons. Phantom Planet announced their hiatus on November 25, 2008.

JJAMZ was a band composed of Greenwald along with James Valentine (Maroon 5), Jason Boesel (Rilo Kiley/Conor Oberst), Michael Runion (solo), and Z Berg (The Like). The group was started at karaoke night at Guys in Hollywood. The band name is an acronym, using the first letter of each member's name. The group was a means of escape from each member's respective band at the time. "JJAMZ started at an interesting time in all of our lives. We all needed some kind of escape from relationships or our other bands. It was a tumultuous time, and the lyrics just came out. It was like word vomit. I can't remember," said Z Berg, previous lead vocalist of The Like, in one of their first interviews as a band. The band played their first concert at the Echo Plex on January 27, 2009. The band released their first and only album, Suicide Pact, on July 10, 2012. It was released through Dangerbird Records.

In addition to the commercials for The Gap and his appearance in Donnie Darko, Greenwald appeared in two music videos for the electronica band M83 – "Don't Save Us From the Flames" and "Teen Angst", the video for Uffie's "ADD SUV" and also in videos for Mark Ronson, including the video for "Just".

Greenwald is in a relationship with Actress and singer Brie Larson. On January 10, 2016 at the 73rd Golden Globe Awards, she thanked him during her acceptance speech for Best Actress in a Drama. She did the same on February 28, 2016 at the 88th Academy Awards, where she won the Oscar for Best Actress. In May 2016 it was confirmed that the pair are engaged.