His father, Charles J. Noth (died March 1966), grew up in Stamford, Connecticut, where he lived with his wife and children in 1962, and was an insurance salesman.
Is incorrectly believed to be purely Irish-American, like his character "Mike Logan" on "Law & Order" (1990). Instead, only his maternal grandmother was Irish American.
Was the last original cast member of "Law & Order" (1990) to leave the series. Although Steven Hill (District Attorney Adam Schiff) is often touted as an original cast member, he did not appear in the series until after its first episode.
Along with Jerry Orbach, Dann Florek and Fred Dalton Thompson, he is one of four "Law & Order" (1990) cast members who later joined the regular cast of one of the spin-off series. He played Detective Mike Logan in "Law & Order" (1990) from 1990 to 1995 and in "Law & Order: Criminal Intent" (2001) since 2005.
His brother made a few appearances on "Law & Order" (1990) as a court clerk. He also has a sister. Both are married.
Became more well-known as Sarah Jessica Parker's on-again, off-again romancer Mr. Big in the TV cable hit "Sex and the City" (1998), after becoming a heart-throb playing Detective Mike Logan in "Law & Order" (1990).
Co-owns The Cutting Room, a lounge and music club in New York City's Flatiron District, with Steve Walter, a Berklee College of Music alumnus. Since opening in 1999, Gwyneth Paltrow, Sheryl Crow, and Norah Jones are among the many people who have performed there.
Has been confirmed that he will return as detective Mike Logan in a guest appearance on "Law & Order: Criminal Intent" (2001) in January, 2005. He last appeared as Mike Logan in the TV movie, Exiled (1998) (TV) in 1998.
He won the World Theater Award 2001- Best Broadway Debut for "Best Man".
Lives with actress Tara Wilson, whom he met while she was working at his bar, "The Cutting Room". 
He and his longtime girlfriend, Tara Wilson, welcomed their son, Orion Christopher, on January 18, 2008 in Los Angeles, weighing seven pounds, ten ounces.