Knole House in Sevenoaks, Kent, was taken over by the charity in 1946 and has battled costly decay problems.
Extensive repairs, part-funded by the National Lottery, have now been carried out in what the Trust described as its largest conservation project to date.
Specialists will also work on precious paintings and furniture in front of visitors in a newly created studio.
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The 15th Century house was formerly an Archbishop's palace and for 400 years was the home of the Sackville family. It also inspired Virginia Woolf's novel Orlando.
Some of the items undergoing treatment include the 17th Century Knole sofa, which inspired copies around the world.
The Royal 'Stool of Easement', an early toilet thought to have been used by Charles II, is also among the collection.
The King's Bedroom, with its suite of silver furniture and bed designed for a visit from James II, now features bespoke lighting and controlled heating systems to protect the collections.
Dame Helen Ghosh, the trust's director general, said: "Knole is one of the Trust's most important houses and this is an exciting moment where we can begin to open new spaces within this vast property to tell the story of its occupants and outstanding collections.
"The new conservation studio is a first-class space for our expert conservators to work on collections from Knole and across the trust, and share their expertise with our visitors."
The project was part-funded by a £7.75m lottery grant.
The future of hundreds of historical objects has been secured as part of a £20m restoration by the National Trust.