In 2000 Cahill performed in
Nicky Silver's
Off-Broadway production of
The Altruists. He was noticed by
Sarah Jessica Parker and shortly after he made several television guest-star appearances including
Sex and the City,
Felicity, and
Law & Order: Special Victims Unit. Cahill was also a recurring guest-star in the
NBC sitcom
Friends as
Rachel's young assistant and boyfriend,
Tag Jones. In 2002, he starred in a short-lived
WB drama,
Glory Days. In the 2004 hockey movie
Miracle, Cahill had the chance to play his boyhood hero, goalie
Jim Craig. He had never played the
goaltender position prior to the movie, so most of the game-action sequences of Craig were filmed with former
NHL goalie
Bill Ranford doubling for Cahill, although Cahill did shoot several key sequences from within goal. When
CBS decided in 2004 to create a third
CSI series,
CSI: NY, to add to its
franchise, Cahill was hired to play the cocky, snarky homicide detective,
Don Flack, who backs up the team of CSIs. The show ran for nine seasons and he was in all 197
episodes. Cahill went back to the
Atlantic Theater Company in June 2011 and performed at the conclusion of its 25th Anniversary season. He was in
Tom Donaghy's one-act play,
I Need a Quote, about "a hilarious telephone conversation between a single mother and a home insurance salesman." In the summer of 2012 Cahill performed in
David Adjmi's play,
3C, at the
Rattlestick Playwrights Theater in New York City. He played "annoying neighbor Terry, a swinging bachelor true to the era, who unlike everyone else in this play, has no deep feelings at all." In 2014, Cahill was cast as a main character starting in the second season of the CBS summer drama
Under the Dome. He played Sam Verdreaux, a former EMT and reclusive brother-in-law to "Big Jim" Rennie. The series was canceled after three seasons. Cahill was cast as the male lead in the 2016
ABC legal drama
Conviction. He played
Manhattan District Attorney Conner Wallace, who creates the Conviction Integrity Unit, which is set up to re-examine cases where there is a credible suspicion of wrongful conviction. The series was canceled after one season. In 2019 Cahill played a charismatic criminal in a three-part
story arc in CBS's
NCIS: New Orleans. Among other crimes he was involved in the death of NCIS Special Agent
Christopher Lasalle. In this series, he reunited with his former
CSI: NY co-star,
Vanessa Ferlito. ==Personal life==