Noth (pronounced with a long "o", rhyming with "both") traveled throughout the United Kingdom, Yugoslavia, and Spain with his mother, former CBS News reporter Jeanne Parr.

After graduating college, Noth starred in the low-budget film Jakarta, and had small parts in films such as Smithereens and Baby Boom and on TV series such as Hill Street Blues and Another World.

In 1988, he filmed a pilot for the dramatic legal/cop series Law & Order. In 1990, NBC began airing the show, which slowly gained recognition from critics and fans. The darkly handsome Noth's intense portrayal of the cocky, troubled Det. Mike Logan made him the most popular actor on the show, so viewers were stunned when producers fired him in 1995.

Frequent personality clashes between Noth and the writers were cited; both sides remained cool toward each other but warmed up long enough to film a Logan-dominated TV movie, Exiled: A Law & Order Movie (1998).

Noth scored a strong comeback in 1998 when he began the role of Carrie Bradshaw's enigmatic on-again/off-again boyfriend "Mr. Big" on HBO's cultural phenomenon, 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.

Noth co-owns a New York City nightclub called The Cutting Room. He often can be found on the NYC party scene, or in the local gossip rags. His long list of female friends includes actress Winona Ryder and supermodel Beverly Johnson, with whom he had an extremely volatile relationship that ended in litigation.

In 2005, Noth returned to the role of Mike Logan. Starting with the 2005-2006 season of Law & Order spinoff Criminal Intent, he has played one half of a detective team which alternates episode-appearances with the original detective team (played by Vincent D'Onofrio and Kathryn Erbe).