Noth, the youngest of three sons, was born in Madison, Wisconsin, the son of Jeanne Parr, a former CBS news reporter, and Charles Noth, an insurance salesman. Noth's father died when Noth was eight. During his childhood, Noth traveled with his mother and brothers throughout the United Kingdom, Serbia, and Spain.

Noth made his film debut in 1988 in the low-budget Jakarta. He also had small parts in films such as Smithereens (1982) and Baby Boom (1987), and on TV series such as Hill Street Blues and Another World. In 1988, he filmed a pilot for the legal/police drama series Law & Order. In 1990, NBC began airing the show, which slowly gained recognition from critics and fans. Noth was fired from the show in 1995. Personality differences between Noth and Wolf settled down long enough to film a Logan-centric TV movie, Exiled: A Law & Order Movie (1998).

Noth scored a strong comeback in 1998 when he took on the role of Carrie Bradshaw's enigmatic on-again, off-again boyfriend "Mr. Big" on HBO's Sex and the City. Noth played the role for six years, during which he also appeared in a small role as Helen Hunt's husband in the hit film Cast Away (2000) and starred in a Broadway revival of The Best Man. In 2001, he played an FBI agent on a couple of episodes of Crossing Jordan, re-uniting him with Law & Order co-star Jill Hennessy In 2005, Noth returned to the role of Mike Logan on Law & Order: Criminal Intent, joining the show in its fifth season following a guest appearance on a fourth-season episode. On this spin-off of the original Law & Order, Noth's detective team alternates episodes with Vincent D'Onofrio and Kathryn Erbe's characters.

Noth co-owns the New York City nightclubs The Cutting Room and the Plumm. He also owns Once Upon A Tea Cup in Windsor, Ontario, Canada, with girlfriend Tara Wilson. They have a son, Orion Christopher Noth, born on January 18, 2008, in Los Angeles.