The former tennis superstar and I'm A Celebrity... runner-up told People that the disease was detected early and that her prognosis is excellent.
Navratilova said: "I cried. It knocked me on my ass, really. I feel so in control of my life and my body, and then this comes, and it's completely out of my hands.
"I went four years between mammograms. I let it slide. Everyone gets busy, but don't make excuses. I stay in shape and eat right, and it happened to me. Another year and I could have been in big trouble."
The cancer was discovered in February when a biopsy came back positive following a routine mammogram, with Navratilova diagnosed with a non-invasive form of the disease confined to the milk ducts.
Navratilova's friend and obstetrician/gynaecologist Dr Mindy Nagle said: "It was the best-case scenario you could imagine for detecting breast cancer."