In 2004, Dennen released his self-titled first album, Brett Dennen. His second album, So Much More, includes the singles "Ain't No Reason", "She's Mine" and "Darlin' Do Not Fear". Dennen also contributed a cover of "Private Life" to the 2006 tribute album Drink To Bones That Turn To Dust: A Toast To Oingo Boingo.
In an interview with American Songwriter magazine, Dennen acknowledged Paul Simon's significant influence on his music as well as commenting that Paul Simon's "“Graceland is my all time favorite album.”
As The Mosaic Project's Resident Rock Star, Dennen created an original musical curriculum for the program, which they released as an album called Children's Songs for Peace and a Better World, in 2003. It won a Children's Music Web 2004 Award and a Parent's Choice 2004 Approved Award.
Dennen was named by Rolling Stone magazine as an “Artist to Watch” and, in 2008, Entertainment Weekly called him one of its eight "Guys on the Rise". He has attended[vague] festivals such as the Rothbury Music Festival in Michigan, Telluride Bluegrass Festival, Lyons Folk Festival, and the Mile High Music Festival in Colorado.
In 2008, Dennen, along with Jason Mraz, contributed the song "Long Road to Forgiveness" to the Survival International charity album Songs for Survival. Earlier in the fall, Dennen toured Australia with Pete Murray. In late October, Hope for the Hopeless, Dennen's third album, was released on the Downtown/Dualtone partnership. The song "Ain't Gonna Lose You" was featured on Grey's Anatomy and topped the iTunes folk charts. The record reached number 6 in the digital charts, and was followed by a successful mini-tour.
Dennen at Newport Folk Festival 2009
On August 1, 2009, Dennen performed at the Newport Folk Festival in Newport, Rhode Island.
In 2010 Dennen played at the Oulipo Ballroom in Kentucky. He played a mixture of Paul Simon covers along with unreleased tracks from his upcoming album.
His fourth album, Loverboy, was released on April 12, 2011.
In 2009, Dennen's song "She's Mine" was featured on the soundtrack for the movie According to Greta; in 2010, Dennen's song "Darlin' Do Not Fear" was featured on the soundtrack for the TV show Parenthood