The 40-year-old Dancing With the Stars co-host told People that it felt like "an elephant was sitting on my chest" when doctors told her that the lump in her breast was cancer.
Harris consulted three different specialists who told her she could get a double mastectomy or second lumpectomy with radiation -- the less radical option.
"I was sick of feeling the way that I did in the days after the diagnosis," Harris said of her decision to get the double mastectomy. "I knew I needed to take control."
Harris is married to Michael Hess. She has two children, Josselyn, 6, and Hillary, 3.
"Thks for the well wishes, kindness & support. It means so much to me & my family! It's a tough road but I'm here to stand up & fight cancer," Harris tweeted Wednesday.
Actress Angelina Jolie also decided to undergo a double mastectomy last year, writing in a New York Times op-ed that she carried a gene, BRCA1, "which sharply increases my risk of developing breast cancer and ovarian cancer."
"Once I knew that this was my reality, I decided to be proactive and to minimize the risk as much I could. I made a decision to have a preventive double mastectomy."