The 59-year-old McCartney proposed Monday during a trip to Britain's Lake District, his spokesman said. McCartney, 59, got down on one knee to propose to Mills, 33. He gave her an Indian sapphire and diamond ring he bought in Jaipur during a holiday in January.
The two said they plan to marry "sometime next year."
McCartney's wife of 30 years, Linda McCartney, died of breast cancer in 1998.
CBS News Correspondent Kimberly Dozier reports McCartney and Mills have been just about inseparable since they met two years ago.
Family and friends were at first very protective, but then happy to see how she helped him recover, after losing Linda. He said that he'd cried for more than a year after her death, and then decided he was ready to get on with life -- and met Heather.
"Paul and Heather said today they would like to thank their relatives and friends for all the great support they have shown them since they met two years ago," said a statement from McCartney's office.
Mills is a former swimwear model whose left leg was amputated below the knee after she was run down by a police motorcyclist in 1993.
The two met at a charity function when Mills appealed for help for her own foundation, The Heather Mills Trust, which provides limbs for victims of war around the world.
Now she works for several charities for the disabled, and that's part of what drew the couple together.
McCartney, whose romance with his first wife was legendary among Beatles fans, said last year that he was smitten with Mills instantly.
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