The National Trust has taken control of Divis and the Black Mountain after four years of planning and preparation.
Work will be done to protect the natural habitat
The area lies on the fringes of north and west Belfast.
It has special historical and cultural significance across all communities and has been bought for about £3m with the help of the government and the Heritage Lottery Fund.
The land, almost 1,500 acres of Divis Mountain, and a large portion of Black Mountain, was owned by the Ministry of Defence.
It has great archaeological significance and there is evidence of Neolithic and prehistoric settlements, including hut circles and monuments.
It is also home to numerous species of birds and wildlife, with slopes covered by a mixture of heath, heather and bog.
But there is work to be done to protect the natural habitat of the landscape before the National Trust opens the area to the public next summer.
It will include securing the site, constructing paths and fences and erecting signs.
Volunteers will be recruited to help out and a community warden appointed.
'Wonderful heritage asset'
National Trust director-general Fiona Reynolds said it was proud to be a member of the coalition which would enable "the opening up of Divis and the Black Mountain to the people of Belfast for the first time".
"The trust was founded on the principle that everyone should have access to open countryside," she said.
Ruth Laird, Richard Rogers, Kevin Baird and Mike Dobson celebrate the deal
"As today proves, this vision is as relevant to today's society as it was 100 years ago.
"We delight in the opportunity that this acquisition gives us to contribute in such a significant way to the people of Belfast and further afield."
Kevin Baird from the Heritage Lottery Fund said the area was a wonderful heritage asset.
"Thousands of people will now be able to visit and enjoy the mountains for the first time and we're delighted to have been able to help make that possible," he said.
The Dobbin family were the last family to live on the mountain.
Family member May Totten said she lived in Divis Lodge and walked across the mountain to school with her brothers Ernie and Alan.
"We would go across the meadows and I would go under barbed wire fences," she told the BBC's Good Morning Ulster.
"When you got to the end of the road your feet were wet and you just had to go on to school the way you were."
Her sister, Minnie Crawford, said the view from the mountain in summer time was fantastic.
"In the winter time, with snow, I remember them out digging the paths to let us walk to get down to work," she said.
"The air on the mountain was terrific - really clean, fresh air. We didn't see doctors - the only time we would have seen a doctor was when mummy was having her babies - it was really healthy to live on the mountain."