After passing the New York State Bar examination Marx enrolled at the BMI Lehman Engel Musical Theater Workshop in order to meet potential clients in the entertainment industry. Here, he met Robert Lopez who was also in the course. Their first major project together, a spec Muppet movie, Kermit, Prince of Denmark, which was very loosely based on Hamlet, won them part of the $150,000 Kleban Award. Together, they created the original concept for Avenue Q and wrote all the show's 21 songs. Avenue Q is currently running Off-Broadway and continues to have various international productions. It is now the 23rd longest running musical in Broadway history. Avenue Q won the 2004 Tony Award for Best Musical. Lopez/Marx's musical score earned them a 2004 Tony Award, and another Tony Award was awarded to Avenue Q bookwriter, Jeff Whitty. The musical's Original Cast Album, on the RCA/Victor label, was nominated for a Grammy Award. To date, it has sold over 150,000 copies. Marx can be seen in the documentary film', which followed the trajectories of four Tony-nominated musicals from 2004, Avenue Q, Wicked, Taboo and Caroline, or Change. Marx, along with filmmaker Dori Berinstein and actor Alan Cumming, provided the audio commentary for the documentary's DVD. Lopez and Marx wrote four songs for a musical episode of the NBC sitcom Scrubs which aired on January 18, 2007. Their song "Everything Comes Down to Poo" was nominated for an Emmy Award. Marx appeared in the episode as a pharmacist, dancing in the background during the song "We're Gonna Miss You Carla". Marx co-wrote the theme song for the Logo Network's animated series ' and contributed additional songs for the show. Lopez and Marx have written songs for the Disney Channel TV series Bear in the Big Blue House and The Book of Pooh, as well as touring stage musicals for the children's theater company Theaterworks/USA. Marx contributed to a musical episode of the NBC sitcom Scrubs, an episode that the New York Times reported "energized a cast and crew that, at a point when most situation comedies are sputtering along or dead, have recently been doing some of their best work." Marx enjoyed working on the show enough that he attributed it as a reason for his move to Los Angeles. In comparing it to his work on Avenue Q, Marx said: "It took us five years to write Avenue Q... There were a million readings and previews and staged readings. With this thing, we wrote the songs in a week. They rehearsed for a week. They filmed it in a week, and it was done. It was liberating, and a collaborative effort that created a much more feel-good way of working." On December 13, 2008, Marx premiered a new song he wrote, "White Kwanzaa", on the CNN show D.L. Hughley Breaks the News. Marx collaborated with Mervyn Warren on a song called "You Have More Friends Than You Know" for the It Gets Better organization. The song was featured on the television program Glee on April 18, 2013. Marx recorded his version of the song and made it available for download with a portion of the proceeds going to support The Trevor Project. Marx has also made a karaoke version of the song available for those who want to perform their own version and help spread the song's message, and has featured other versions on the song's website. In 2015, the Broadway musical titled Home Street Home premiered, which was co-written by Marx with Fat Mike of punk band NOFX, and activist and dominatrix Soma Snakeoil.
Other work
Marx has also written the book How To Win A High School Election.
