In a video clip that appears on the cancer advocacy group's Web site, Nixon, 42, said she became an ambassador not only because she is a survivor, but because she is the daughter of a survivor.
"Cynthia's personal story will certainly help draw attention to the impact of the disease as she joins the global breast cancer movement and our millions of survivors and activists working daily through Susan G. Komen for the Cure to create a world without breast cancer," said the group's president and CEO, Hala Moddelmog, in a statement.
In her own words as to why she waited to announce her health news, Nixon said in an ABC interview that aired on both Good Morning America and Nightline: "I didn't really want to make it public while I was going through it. I didn't want paparazzi at the hospital, that kind of thing,"
Nixon's DiagnosisDiagnosed while starring off-Broadway in a revival of The Prime of Miss Jean Brodie, Nixon told ABC, "I go for my completely routine mammogram and then I get a call from my gynecologist. And she says, 'Well, I have some – it's not such great news, but here it is, but it's very small and we're just going to get in there and take it right out, right away, and then you'll probably have radiation.'
"I felt scared. And I thought, 'Oh, I don't want this to be happening.' I was very cognizant of if it's going to happen, this is the best way for it to happen, that it's found so early and we can just get right on it."
Nixon's own mother, Ann, successfully battled breast cancer twice, the first time when her daughter was 12. "I always sort of thought, 'I'm probably going to get breast cancer. There's a really good chance,' " Nixon said.
Telling Her ChildrenSurgery took place on a Sunday, so Nixon would not miss a single performance of her play.
"My girlfriend was very scared," Nixon said of her partner of four years, Christine Marinoni. "She was in a panic. She was just trying to calm herself down any way she could. And, actually, we made a big point of talking to my kids about it."
Nixon has two children – daughter Samantha, born in 1996, and son Charles, born in 2002 – with her former partner, from 1988 to 2003, English professor Danny Mozes.
"I talked to them together and, basically, I told them, 'You know, they found some breast cancer in my right breast. It's very small. It's very early. I'm going to have an operation. They're going to take it out, and then we're going to have six-and-a-half weeks of radiation every weekday, and this is like what Grandma went through and I'm going to be fine," said Nixon.
As for any special regime she might be undertaking since her treatment, Nixon told PEOPLE last week, at a Point Foundation scholarship benefit in New York, "I started doing Gyrotonics in January, and I really love it. Me and my family went to a Yoga retreat in Jamaica. We do yoga in the morning and then go to the beach."
As for some of the advice she dispenses on the Komen Web site, Nixon says, "Talk with your doctor, make healthy lifestyle choices and most importantly, know your body – as that can make all the difference in the world."
With additional reporting by JEFFREY SLONIM