The 33-year-old actress announced the news Thursday on her Who Say page. Mitchell posted a Throwback Thursday photo of herself and Cameron, and included the caption, "#tbt to the place where Michael asked me to marry him, and this past year where we stood as a family of three! Can't believe in 2015 we will be a family of 4!!!"
Mitchell and Cameron are already parents to one-year-old daughter Kenzie Lynne. The couple have been together for over a decade, and married in October 2008.
"Life has been flipped upside down in the most beautiful way," Mitchell said of Kenzie to OK! magazine in February. "I couldn't be happier. My husband and I pinch ourselves every day because she's such a gift. She's brought so much joy and light to our lives."
Mitchell is best known for her role as Lucy Camden-Kinkirk on 7th Heaven. The series aired from 1996 to 2006 on The WB, and ran for a final season on The CW. The actress last appeared in The Dog Who Saved Easter, and is scheduled for Toxin later this year.