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.

Marx (and his parents) can be seen in the documentary film ShowBusiness: The Road to Broadway, 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 (with Debra Fordham) four songs for a musical episode of the NBC sitcom Scrubs which aired on January 18, 2007.

The New York Times reported that the episode "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.

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.

[4] Marx was an original collaborator with Lopez and South Park creators Trey Parker and Matt Stone on the 2011 Broadway musical The Book of Mormon.

It was co-written by Marx with Fat Mike of punk band NOFX, and activist and dominatrix Soma Snakeoil.