Cynthia Nixon was born in New York City, New York, the daughter of Anne Knoll, an actress, and Walter Nixon, a radio journalist. She began acting at age 12 as the object of a wealthy schoolmate's crush in The Seven Wishes of a Rich Kid, a 1979 ABC Afterschool Special. She made her feature debut in Little Darlings (1980).
Nixon graduated from Hunter College High School, and made theatrical history while a freshman at Barnard College in 1984, simultaneously appearing in two hit Broadway plays directed by Mike Nichols. The two theaters were just two blocks apart and Nixon's roles were both short, so she could run from one to the other. That year's Oscar-winning Best Picture Amadeus, directed by Milos Forman, also featured her in a brief role as Mozart's young servant-girl.
Nixon was a founding member of the theatrical troupe The Drama Dept.
She raised her profile significantly as one of the four regulars of HBO's successful comedy Sex and the City (1998-2004), as the no-nonsense lawyer Miranda in support of series star Sarah Jessica Parker. After Emmy nominations as Outstanding Supporting Actress in a Comedy Series in 2002 and 2003, Nixon took home the trophy in 2004 for the series' final season. The immense popularity of the series led Nixon to enjoy her first leading role in a feature.
In 2006, Nixon won the Tony Award for Best Actress in a Leading Role (Play) for David Lindsay-Abaire's Pulitzer Prize-winning drama Rabbit Hole. In 2008, she revived her role as Miranda Hobbes in Sex and the City feature film, directed by HBO executive producer Michael Patrick King and co-starring the cast of the original series.
Nixon has two children, daughter Samantha (b. 1996) and son Charles Ezekiel (b. 2002), with Danny Mozes, an English professor, with whom she had a relationship from 1988 to 2003.
Nixon began dating education activist Christine Marinoni in January 2004; media reports of the relationship started surfacing in September of the same year. In February 2005, the New York Post and other sources reported that Nixon had moved to Brooklyn to live with Marinoni.
Discussing her relationship in an interview in New York Magazine in 2006, Nixon stated that she never felt any struggle with her sexuality:
"I never felt like there was an unconscious part of me around that woke up or that came out of the closet; there wasn't a struggle, there wasn't an attempt to suppress. I met this woman, I fell in love with her, and I'm a public figure."
In an interview in May 2007, she said.
"In terms of sexual orientation I don't really feel I've changed... I'd been with men all my life, and I'd never fallen in love with a woman. But when I did, it didn't seem so strange. I'm just a woman in love with another woman."
In March 2008, Fox News reported that Nixon has been in a relationship with Marinoni since 2003.
In April 2008, she received an award from the Point Foundation, which provides scholarships to gay students in the U.S., for being a role model for young gay people.
In an interview with Good Morning America that aired on April 15, 2008, Cynthia Nixon announced for the first time that she battled breast cancer, after being diagnosed during a routine mammogram in October, 2006. Initially she did not go public because of the stigma involved, but since then, she not only has openly admitted that she had cancer, she has become a breast cancer activist and was able to convince the head of NBC to air her breast cancer special in primetime.
In 2008, she began to serve as Ambassador for Susan G. Komen for the Cure.