He is known for his television roles as NYPD Detective Mike Logan on Law & Order (1990–1995), Big on Sex and the City (1998–2004), and Peter Florrick on The Good Wife (2009–2016).

[7][13] Noth said that he started taking LSD with friends at age 15, once walking into someone else's house in Newport Beach while high and jumping naked off their pier into the water.

"[11] He was fired from a number of restaurants, once for forgetting to return Governor Hugh Carey's credit card with the bill, and settled into cater-waitering bar mitzvahs and weddings.

[25][27] In 1984, Frank Rich of The New York Times wrote that of the supporting cast, only Noth's and Ray Aranha's performances "leaves firm impressions" in the world premiere of the Wole Soyinka political satire A Play of Giants at Yale Repertory Theatre, where Noth played a sculptor creating a portrait of African dictators gathered at a United Nations embassy.

[37] In 1985, Noth acted in Keith Reddin's Rum and Coke at Yale Repertory Theatre, a play about the orchestration of the Bay of Pigs Invasion.

[22][34] In 1986, while working on the TV series Hill Street Blues in Los Angeles, Noth heard that Zoe Caldwell would be directing Hamlet at the American Shakespeare Festival at Stratford, Connecticut and successfully auditioned for the title role.

[41][42] In April 1989, Noth played "bohemian--out of place, angry with the world" Frank Shabata in Darrah Cloud's adaption of the Willa Cather novel O Pioneers!

[46] The New York Times wrote "the actors are so good that they may have put more flesh on the characters than even Mr. Linney intended" and that Noth and co-star Dana Reeve were "amusingly synchronized as they purred in unison...to the strains of Schubert".

[49] In 1998, while working on Sex and the City before its TV debut, Noth did his first radio play as fortune-hunter Morris Townsend in the Voice of America production of The Heiress, an adaptation of the Henry James novel Washington Square, opposite Amy Irving in the title role.

[50] In 2000, Noth made his Broadway debut[51] in a revival of Gore Vidal's 1960 play The Best Man at Virginia Theatre as the conniving Senator Joseph Cantwell.

[55] In the 2002 premiere of Christopher Shinn's play What Didn't Happen at Playwrights Horizons, Noth's portrayal of Peter was described as "an enjoyably robust portrait" by The New York Times and "an endearing, minor-key star turn" by Variety.

[61] Noth received glowing reviews as petty criminal "Teach" in David Mamet's play American Buffalo at the 2005 Berkshire Theatre Festival.

[36][62] In 2008, Noth portrayed Paul Zara in Beau Willimon's Off-Broadway debut play Farragut North staged by the Atlantic Theatre Company.

[63][64] The play had its world premiere in the week after the 2008 United States presidential election and The New York Times critic Ben Brantley wrote that he "enjoyed Mr. Noth's weary, bluff, stiff-jointed Paul.

[66][67] Sex and the City 2 director Michael Patrick King demanded Noth lose the weight he gained for his role in the play before filming began.

Noth filmed a pilot for the legal/police drama series Law & Order in 1988, playing NYPD homicide detective Mike Logan.

[78] In the 1997 television mini-series Rough Riders on TNT he portrayed Craig Wadsworth, a member of the 1st United States Volunteer Cavalry.

On this spin-off of the original Law & Order, Noth's detective team alternated episodes with Vincent D'Onofrio and Kathryn Erbe's characters.

[85][86] In the 2005 rom-com film, The Perfect Man, Noth's portrayal of the romance expert from whom Hilary Duff's lead character models a secret admirer for her mother was described as "appealing in a thinly written role" by Variety.

[100][101] In 2016, Noth joined the third season of the FX/Fox 21 Television Studios produced series Tyrant in the regular role of U.S. General William Cogswell who offers military support to the interim president of a fictional Middle Eastern country that is trying to start a social democracy.

[15][102][103] A review of Noth's first Tyrant episode likened General Cogswell to his Peter Florrick character in The Good Wife.

[103] In an interview before the episode aired, Noth said he was "pretty much done with parts that resemble Mr. Big or Peter Florrick",[104] he was turning to darker roles after years of playing (mostly) good guys on his three hit TV shows.

[105][15][106] In the 2016 film Chronically Metropolitan Noth's portrayal of a philandering professor/novelist was praised by The Hollywood Reporter for "infusing his familiar-feeling character with intriguing nuances" and as "very good" by the Los Angeles Times.

[112][113][114] The New York Post wrote in its review of the show that "Noth, who's always reliable, is fine here, but doesn’t have much to do other than set up each storyline and then bark lots of orders.

"[114] In 2018, Noth played Jack Robertson in the Doctor Who episode "Arachnids in the UK" and returned to the show in 2021 New Year's Day special, "Revolution of the Daleks".

[115][116][117] In 2021, Noth played the lead role of William Bishop in CBS reboot crime drama series The Equalizer, which was written by Andrew W. Marlowe and Terri Edda Miller.

[13][121][3] He also co-owned the New York nightclub The Plumm with Noel Ashman and other investors including David Wells and Damon Dash.

[142] On December 18, 2021, another woman accused Noth of sexual assault, in New York in 2010, saying that when she was 18 he forcibly kissed her and removed her tights in an effort to digitally penetrate her.