The actress, known for her role as Miranda Hobbs in the hit series, made the announcement yesterday at a gay marriage rally in New York.
The 43-year-old was pictured wearing an engagement ring at the event, which she attended along her co-star Kristin Davis.
She began dating Christine, an education activist, in 2003.
Last year Cynthia said she and Christine would only get married if it became legal in New York.
She said then: "I don’t really want to get married, to get married pretend."
“I think we’d like to do it in a real, actual, legal way that the state would recognize."
The actress has two children from her marriage to Danny Mozes, which ended shortly before she began dating Christine.