The 39-year-old star of "Samantha Who?" and her fiance, Dutch bass player Martyn Lenoble, 41, managed to keep the news that they had become parents under wraps for five days, People.com reported.
"Actress Christina Applegate and fiance, musician Martyn LeNoble, welcomed daughter Sadie Grace LeNoble on Thursday, Jan. 27, 2011, in Los Angeles," Applegate's spokesperson said in a statement.
"Mother and daughter are doing great," the rep added.
"She's a beautiful little girl, and they are so happy and in love with her," a source close to the couple told the website.
Applegate was diagnosed two years ago with breast cancer and was public about her decision to have a double mastectomy to prevent the return of the disease.
The actress told the website last year about what kind of parent she anticipates being, "I'll probably be a little bit hippie and a little bit Type A," she said.
"I'll take from what my mother did, which was way hippie and like, 'Do what you wanna do,' and bring some things that I know from watching my friends raise their kids," she added.