The 44-year-old former model told The Sun newspaper the growth was found after she suffered a recent ski injury.
"An MRI scan picked up a tumor in the top of my left leg, which is the same one I had amputated below the knee," Mills said. "Doctors said it had been there for some time. Thankfully, it was all fine. They told me the tumor was benign."
The Sun did not say what treatment Mills has undergone, or plans to undergo, for the tumor but the newspaper said she still plans to race for the British Disabled Ski Team in Austria this week.
Part of Mills' leg was removed in 1993 after she was hit by a police motorbike.