Marinoni gave birth to Max Ellington Nixon-Marinoni on Monday, Nixon's rep confirmed to the magazine.
"Christine and baby are doing great," her rep told People.
Nixon announced in 2009 at a rally for marriage equality in New York that she and Marinoni were engaged. Nixon has been at forefront of the fight for marriage equality in the state for the past few years.
The Sex and the City star has two children, Samantha, 14, and Charles, 8, from her previous relationship with photographer Danny Mozes.