After graduating from high school in 1982, Eigenberg enlisted in the United States Marine Corps Reserve, serving for three years (1982–1986), and was honorably discharged at the rank of Lance Corporal.

In 2002, Eigenberg starred as the short-lived character Officer Ross in the Season 3 episode of Third Watch, entitled “Superheroes: Part 1”. In July 2004, he appeared in the sci-fi series The 4400. He appeared as Carl Morrissey in part two of the pilot, entitled “The New and Improved Carl Morrissey”. Eigenberg appeared as a suspect in the sixth episode of the TV series Raines, entitled “Inner Child”, which first aired on April 20, 2007.

He appeared in an episode of Ghost Whisperer, alongside Jennifer Love Hewitt, and in CBS’s NCIS as Ted Bankston, a former NSA analyst who was suspected of mishandling classified information, which was eventually proven to be true. His character had kidnapped Michelle Lee’s (Liza Lapira) daughter/sister, Amanda, and had forced her to work for him; he was killed off during a stand-off between himself, Lee, and Leroy Jethro Gibbs (Mark Harmon) in the Season 6 episode, “Dagger”. Eigenberg had a recurring role in the HBO series Sex and The City as Steve Brady, Miranda Hobbs’ on-and-off-again boyfriend and eventual husband. In 2009 he appeared in Season 6 episode 13 of Cold Case titled “Breaking News”. He played the 1988 version of Nathan Kravet.

In 2010, he appeared as Agent Russell Goldman in the Season 5 episode of Criminal Minds, entitled “Parasite”. In March 2011, he appeared in Season 3 of Castle, in the episode “One Life to Lose”, as Peter Connelly. He appeared in the May 16, 2012 episode of Law & Order: Special Victims Unit, entitled “Strange Beauty”. In the fall of 2012, Eigenberg joined the cast of NBC’s Chicago Fire as Senior Firefighter/ Lieutenant Christopher Herrmann.