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". He is also a member of another Los Angeles band, Blackblack. At live shows he is known for 1980s styled dance moves, which he got from watching David Byrne of the Talking Heads.
His band Phantom Planet is best known for producing the song "California", which was used as the theme song of the teen drama "The OC", which lasted 4 seasons.
Greenwald's mother is a large part of his musical education. "Music has just always been part of my life. My mom is a guitar teacher and an abstract painter -- an all-around artist and a wonderful woman. I would listen to records with her and take guitar lessons from her. Music has been an appendage almost. It speaks my language."
Greenwald was only a young toddler when his mother built her own harpsichord from a kit. It was his first memorable interaction with a musical instrument, and Greenwald was wholly fascinated.
"Why is my mom spending so much time away from me and building a musical instrument? It's probably something extremely special," Greenwald said.
As he grew older, Greenwald began to fiddle around with the piano by himself. "I was probably 4 or 5. Before I even really was interested in playing the keys, I noticed that the mute pedal and the sustain pedal on our piano would make noises, reverberating around inside of the wood of the piano if you stepped on it," he said. "I would stop those pedals to make a 'choo-choo' sound inside the piano -- that was probably my first musical composition."
Greenwald's mother then taught him to play guitar around age 10. He learned to play "California Girls" and became a huge Beach Boys fan. A year later he started writing songs. "I tried to write something similar, something about how summer was ending - a song complaining about having to go back to school."
Greenwald is also a vegan and has appeared in at least two ads for PETA. He told PETA his reasons for going vegan. "In college I stopped eating red meat on a bet with my girlfriend at the time. She bet she could stop smoking, and I bet that I could stop eating red meat. She started smoking again, so I won, which I'm always proud of.…I had been a vegetarian as a child for whatever reason. I guess kids sometimes follow their instincts…My friend dared me to go vegan as I was reading this book Fast Food Nation, which opened my eyes to a lot of cruelties.…Ever since then, I've been vegan and enjoyed it daily."
When speaking of his lyrics, Greenwald says, "I think that every song is sort of like a study in the way that a song should be. Each one is a feeling or a sentiment or a time or a place or all that stuff combined."
Greenwald has also promoted Apogee's Duet recording system, which he uses to record demos. Some of these demos cn be found on his solo Myspace project called Family.