"There's a big contrast between regulating about 100 clinics involving some of the richest doctors in the country and regulating 190,000 charitable bodies of such fabulous diversity," she said.
"I was also attracted by a fresh start with a new legislative framework, which is in view with the Charities Bill. I enjoy the challenge of something new."
Leather is also interested in championing the public interest in charities, which was made a specific objective of the commission under chief executive Andrew Hind and her predecessor Geraldine Peacock, and in engaging more proactively with charities.
"I'm not talking about beating a drum for charities," she said. "They and the umbrella organisations do that well enough themselves.
"I'm talking about good regulation being in the interests of the sector, indirectly helping it to be what it is - free to think and come up with fresh ideas, and more value-driven."
She thinks she is well qualified to do the job by the variety of her experience. "The values I've expressed in my professional life include consumer involvement, the importance of public debate, and regulation in the interests of the sector being regulated," she said.
"Before my chairing roles I spent time as a campaigner, when most of my concern was the relationship between poverty and nutritional and health inequality. That's combined with regulation at the HFEA and being deputy chair at the Food Standards Agency, dealing with the country's largest manufacturing sector."
Another reason she likes the voluntary sector is that it's "at the sharp end of public debate". As an example she mentions the School Food Trust, which she chaired and which attracted the sector's criticism last year, when the Government announced that £45m of its £60m funding would come from the Big Lottery Fund.
"The standard of school food was not just about Jamie Oliver," she said.
"It was about charitable organisations doing the hard work on this in the 1990s, so that when ministers wanted to run with it it was possible for them to do so."
Leather, 50, is married with three teenage children. She lives in Exeter and is a member of the Labour Party and the Christian Socialist Movement.
Her career in public life began in 1997, when she became chair of her local NHS trust. She was made a dame in 2005.
She has a three-year contract and will be paid £50,000 for two days a week - "but the reality is that it's always more than that", she said.
'I try to do a good job. I drive myself hard to get it right'
'The Human Fertilisation and Embryology Authority was challenging and satisfying. It has not been an ordinary few years'
THE PRESS SAYS
'Dedicated, hard-working and very sharp' - Sarah Boseley, The Guardian
'She says fertility clinics should produce babies one at a time. Bilge - she's wrong for the job' - The Sun editorial.