During their time in California, the group progressed to the LA club scene, where they added three members and changed their name to "Train". The additions included Scott Underwood on drums, Charlie Colin on bass, and Jimmy Stafford on guitar. The group gained popularity from 1994 to 2006, during which Monahan and the group released four studio albums. In 2002, the group earned two Grammy awards for songs, including "Drops of Jupiter (Tell Me)", which was inspired by Monahan's mother who had recently died of cancer.

Monahan and his group, Train, took a break from recording and performing between 2006 and 2009. On September 18, 2007, Monahan released his solo album, titled "Last of Seven". After the album's release, Monahan began a global tour to promote the album, including a hit single titled "Her Eyes", which reached the top of Billboard's Hot AC Chart. The song marked Monahan's first major solo single.

During his break from music, Monahan considered getting into acting and was presented with an opportunity for a small acting role in the CSI: NY episode "Second Chances" with his fellow bandmates. Monahan played the former homeless drug addict, Sam Baker, who dated Debbie Fall, portrayed by Kim Kardashian. Kardashian's character became involved in a murder investigation, and during the episode, Monahan performed two songs with his group from their new album: "Hey, Soul Sister" and "Calling All Angels."

Despite being busy with his band, Monahan found time to help and collaborate with other artists. He even recorded a duet of "Marry Me" with Martina McBride and performed with the rock group "INXS." His first performances with "INXS" were on "The Tonight Show" with Jay Leno and then at a concert at Château Ste. Michelle. He also appeared on the show "The Voice," where he performed "Drops of Jupiter" with one of the show's contestants, Vicci Martinez.

He started a podcast in 2013 called "Patcast," where he interviewed various people in the music industry and talked about his experiences with his band "Train." On April 4, 2010, he performed with "The Hollies" at their own Rock and Roll Hall of Fame induction ceremony, where Monahan sang the lead vocals on the song, "Long Cool Woman in a Black Dress." In 2011, he contributed to a cover of Buddy Holly's "Maybe Baby" for the tribute album, "Listen to Me: Buddy Holly," which was released on September 6, 2011.