Robach underwent an on-air mammogram screening last month as part of the morning program's "October Pink Initiative" to raise awareness about the importance of the early detection of breast cancer, People.com said.
"I was a little reluctant at first," Robach told "GMA" co-anchor Robin Roberts, a breast cancer survivor. "I'm 40, I'm the age and I've been putting it off."
Robach, who has no family history of the disease, said after the initial on-air mammogram, doctors followed up with additional tests and last week informed her she has breast cancer.
"It's still hard for me to say the words out loud. I have breast cancer," Robach said.
"Telling my children was the toughest part," she said. "But that's when you get strong, because you have to be strong for other people. ... I'm so grateful that I got the mammogram that day. Robin's words still echo inside of me. 'If I got the mammogram on-air and saved one life then it's all worth it,' she had said. It never occurred to me that that life would be mine."