Harris, 40, told "ET" that numerous doctors told her that the lump in her breast was benign, but she refused to believe it. She finally saw a specialist, who gave her the bad news.
She said her initial response wasn't "Why me?" Instead her next thought was her daughters.
"My daughters are three and six, you know. All they're concerned about is to have mommy active, be able to hold them, hug them, kiss them," Harris said during her interview with "ET."
Harris, who lost her father when he was 50, said she chose to have a double mastectomy "to take control" and made the choice for her family.
"My dad wasn't able to walk me down the aisle," she said. "I will be here to plan my daughter's weddings and they better like the dress I pick."
Harris co-hosted "DWTS" from 2006 to 2010, before Brooke Burke-Charvet replaced her.