Nixon and Marinoni became engaged three years ago at a rally to support same-sex marriage in New York, People reported.
"On May 27, 2012, Cynthia Nixon and her girlfriend, Christine Marinoni, were legally married in the state of New York," Nixon's representative said in a statement. "Nixon wore a custom dress by Carolina Herrera."
No further details on the wedding have been released.
The couple waited until marriage was legal last summer in New York to get married.
"I'm enjoying being engaged very much," Nixon said in 2010. "I don't mind a long engagement, which this one is surely turning out to be."
The couple, who have been together since 2004, have one son, Max Ellington Nixon-Marinoni, who was born in February 2011.
Nixon also has two children from her previous relationship with photographer Danny Mozes.