Robach, who is 40 years old, was too scared to have mammogram. She finally agreed to get one when a producer at "Good Morning America" asked if she would be willing to do it on-air on Oct 1.
A few weeks after the mammogram, she found out she had breast cancer.
"Words I never expected to hear. I was told that I have breast cancer," Robach said on "Good Morning America."
Robach said she will have a bilateral mastectomy on Thursday, followed by reconstructive surgery.
"Only then will I know more about what that fight will fully entail, but I am mentally and physically as prepared as anyone can be in this situation," she said.
She had put off getting a mammogram for more than a year. Robach said her doctor told her the mammogram saved her life.
"I got lucky by catching it early, and there are so many people to thank for making sure I did. Every producer, every person who urged me to do this, changed my trajectory," she said.
She said she hopes her story will inspire women to get mammograms or take a self exam because it can mean the difference between life and death.
Robach was a frequent fill-in on ABC's morning show while Robin Roberts was fighting a serious blood and bone marrow disease.
ABC News contributed to this report.