While progressing from the SF coffeehouse circuit to the LA club scene, Monahan and Hotchkiss added Jimmy Stafford (guitar), Charlie Colin (bass), and Scott Underwood (drums) to their lineup, thus officially forming Train.

In 2002, the band earned two Grammys, one of them for the song "Drops of Jupiter (Tell Me)", which was written by Monahan and inspired by his late mother, who had died of cancer.

On Last of Seven, Monahan duets with folk rock musician Brandi Carlile, with special guest appearances by Richie Sambora and Graham Nash.

That same year, Monahan and some of his fellow Train bandmates took on small acting roles in the CSI: NY episode "Second Chances".

Monahan played the former homeless drug addict Sam Baker (dating Debbie Fallon, portrayed by Kim Kardashian) who becomes involved in a murder investigation.

He also performed several times with INXS during 2011; first on The Tonight Show with Jay Leno and then at a concert at Chateau Ste Michelle.

[7][8] Monahan has sung the U.S. National Anthem at various events: He inducted Journey into the Rock and Roll Hall of Fame on April 7, 2017.

[13] In 2001, Monahan appeared on the Celebrity Top of the Charts Edition of ABC's hit TV game show Who Wants to Be a Millionaire, winning $US125,000 for Camp Ronald McDonald.

[14] In early 2013, Monahan played the role of Neil Redding in a Season 3 episode of CBS's Hawaii Five-0, titled "Paʻani" ("The Game"), which aired on February 18, 2013.

[15] In 2009, Monahan appeared in episode eleven of the sixth season of CSI: NY, alongside Kim Kardashian.

In 2021, Monahan starred in Christmas In Tahoe, a Hallmark Channel film based on Train's album of the same name, and was also executive producer for the movie.