Seaton Delaval Hall, its gardens and the majority of its contents were accepted by the Government in lieu of inheritance tax and gifted to the National Trust through the Acceptance in Lieu (AIL) scheme.
Owner Lord Hastings was faced with an inheritance tax bill of £4.9m when both his parents died in 2007 and he planned to sell the Grade I-listed hall.
But the National Trust launched an appeal in July 2008, which has raised £3m in public contributions to help save the hall.
An endowment fund -- comprising £6.9m invested by the National Trust -- has been set up to care for the property and gardens.
Culture and tourism minister Margaret Hodge joined National Trust director general Fiona Reynolds to celebrate the success of the plan.
Reynolds said: "This is a wonderful Christmas present for the nation. It has been an incredible journey and we have been thrilled by the goodwill and support we've received from so many people.
"From the start of the campaign we've worked together with the public -- and especially the local community -- to shape Seaton Delaval Hall's future and decide how the building, gardens and grounds can best be used for the benefit of everyone."
She added: "Over 100,000 people took part in the biggest public consultation in the National Trust's history and they showed us overwhelmingly that they wanted the National Trust to secure Seaton Delaval Hall's future."
She concluded: "I pay a warm tribute to Lord Hastings for giving us the time to put this package together and in particular to his parents whose love of the hall, gardens and estate ensured their survival into the 21 century. We also thank everyone who has helped us -- from the highest echelons of government to the children in the local school."
The trust plans to open Seaton Delaval Hall to the public in spring.
Plans include restoring the Walled Kitchen Garden through offering gardening plots to schools, other community groups and visitors.
Catherine Atkinson, the National Trust's new property manager for Seaton Delaval Hall, said: "We want to open the doors as soon as possible to give those who've supported the appeal the chance to visit, get involved and share our excitement about this wonderful place."
Seaton Delaval Hall in Northumberland was built between 1718 and 1731 by Sir John Vanbrugh, architect of Blenheim Palace and Castle Howard.
Investments to help the plan proceed included £1m from One North East, the regional development agency for north-east England and £100,000 from Northumberland County Council.
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