The singer – who was a finalist in the 2010 series of the singing competition – was praised by viewers as she showed off her slimmed down frame.
Admitting her decision to lose weight stemmed from a string of health woes, Mary admitted her weight loss has improved her health significantly.
Listing the illnesses she has fought, the former Tesco checkout worker revealed she no longer takes tablets for high cholesterol or high blood pressure.
Mary, who suffers from arthritis, also enthused that her lighter size has eased up the pain in her knees.
"Losing four-and-a-half stone has made a huge difference to my whole body," she explained.
"When I was on The X Factor I was diagnosed with an under-active thyroid. I was sick all the time and they couldn't work out what was wrong with me."
"For years I used to say I'm happy with what I am, but the problems start as you get older and that's what I found.
"I have chronic arthritis in my left knee, I have a bandage on now."
After fearing for her health, Mary signed up to star in Celebrity Operation Transformation which saw her shed a stone in just one month.