Mills, the former wife of Beatles legend Sir Paul McCartney with whom she has six-year-old Beatrice, also said she wanted to inspire others who, like her, use prosthetic limbs.
The businesswoman, who is the patron of the Hunts Point Alliance charity which helps impoverished children in The Bronx, New York, is among 14 celebrities to compete in the ITV1 show.
At the launch yesterday, she said: "The charity's been suffering a lot, I've been funding it always and they said they'd give us a good fee for the charity, so I thought that would be good.
"The other reason I did it is, I've got 6,000 amputees on my website who look to me for inspirational things to do, jumping out of planes or skiing down a mountain, so I thought this would be a really good one to try."
Mills lost part of her left leg after being hit by a motorcycle in 1993 and has used prosthetics ever since.
She and McCartney split in 2006 and divorced two years later after a high-profile court case in which Mills was awarded an estimated 24.3 million.
She said her ex-husband had "lots of support" for her decision to enter the competition.
"My daughter is so excited, she's already got a matching little leotard made up to match me and she's just learned to ice-skate.
"Now I can take her ice-skating and she's not going to have a disabled mum that can't do anything with her," she said.
Mills added that she was not worried about being voted out by the public.
"To be honest, I don't know what the public perception is; I just know what the media perception is. What I know is that I've had nothing but people coming up to me in the streets and telling me, 'We're there for you, we're rooting for you'," she said.