Speaking during her one-woman stage show 30 Million Minutes on Thursday night, Dawn told the crowd that she was told to lose weight in order to have a hysterectomy.
Dawn said her doctor had been convinced she had uterine cancer prompting her to decide to have an operation to remove her womb.
Before undergoing the surgery Dawn dramatically shed 7.5 stone, bringing her weight down from 19 stone to 11.5 stone.
"When I was due to have my hysterectomy the doc told me that if I could lose some weight before the op, they would be able to do it via keyhole, and I would recover in three weeks or so," she said. "Otherwise it would be big open surgery, and three months to recover. So, I set about dropping a few stone.
"No magic wand," she added. "Just tiny, joyless low-cal eating and lots more walking for weeks and weeks. It was grim. I lost seven-and-a-half stone. I could have the keyhole surgery. Great. That’s all it was, practical."
Dawn with Jennifer Saunders in 2009
During her show Dawn also spoke frankly about the highs and lows she has experienced.
The 56-year-old, who divorced her comedian husband of 25 years Lenny Henry in 2010, is now happily married to charity boss Mark Bignell.