He is best known for his role of Tom Collins on Broadway in the musical Rent and performed on television as NYPD Detective Ed Green on Law & Order, Captain Joe West on The Flash, and professor Alec Mercer on The Irrational.

In 1996, Larson's musical Rent took the theatre world by storm, with Martin in the role of gay computer geek/philosophy professor Tom Collins.

[4] He took part in a one-night-only reading benefit of Romeo and Juliet to celebrate the 50th anniversary of the opening of the Delacorte Theater in Central Park, alongside Meryl Streep, Kevin Kline, Raúl Esparza, and others on June 18, 2012.

Ally McBeal's creator, David E. Kelley, attended Rent's Broadway premiere and remembered Martin when the show needed a new boyfriend for the title character.

While still shooting Ally McBeal, Martin heard rumors that actor Benjamin Bratt planned to leave the cast of Law & Order.

Martin had tried out for the show years before and won the minor role of a car-radio thief named Earl the Hamster, but decided to wait for a bigger part.

Wolf hoped to cast him, and upon hearing that CBS and Fox both offered Martin development deals, he gave the actor the part without an audition.

Altogether, he was the fifth-longest serving cast member behind S. Epatha Merkerson, Sam Waterston, Jerry Orbach, and Steven Hill.

On September 14, 2012, NBC announced that Martin would be joining the cast of Smash during season two for a nine-episode arc as Scott Nichols, the artistic director of the Manhattan Theatre Workshop.

On March 4, 2013, it was announced that Martin would replace Lenny Kravitz as Marvin Gaye in Sexual Healing, an upcoming biopic directed by Julien Temple, and produced by Vassal Benton and Fred Bestall.

[14][15] Martin had been attached to the Marvin Gaye biopic for years and had stated that it was his dream role to portray the legendary Motown singer.

[3][20] Martin sits on the board of trustees for the Jonathan Larson Performing Arts Foundation along with Rent co-producer Kevin McCollum.