Bedwellty House and Park in Tredegar, Blaenau Gwent, was home to the town's ironmasters and its civic centre.
Work began in October 2009 and on its completion in 2011 the house will serve as the council's new registry office.
"When fully restored it has the potential to be one of our finest attractions," said the council.
Bedwellty House was built in about 1800 as the mansion of the owners of Tredegar Ironworks and is regarded as one of the most complete ironmaster's residences in south-east Wales.
It was handed over for public use by Lord Tredegar a century ago.
The process of planning and bidding for grants and funding for restoring Bedwellty took almost four years, with the people of Tredegar and local organisations being consulted on the key features being planned.
The Regency building, where Aneurin Bevan, the late National Health Service founder and Ebbw Vale MP launched his political career, will become a tourist attraction and serve as a heritage centre with town and community facilities.
The house's internal structure, including its ornate plaster ceilings, has been renewed as well as restored in its authentic style, said the council.
Sash windows and shutters which had been out of use for 50 years have been returned to working order.
Wherever possible features from floorboards to ceilings are being renewed using authentic materials and craft skills.
The estate's parkland and formal gardens are also being refurbished and conserved.
Its fountains, cascades and ponds are bring brought back to life, as is its walled kitchen garden, rare ice-house, Chinese-style limestone grotto and long shelter - built in 1908 and used as shade while watching croquet and tennis.
Jason Owen, the council's executive member for leisure and culture, said the  renovation project was "a huge step forward" for the regeneration of the town and of Blaenau Gwent.
"In partnership with local people we have saved one of the most important heritage landmarks in the Valleys," he said.
"Bedwellty House represents the heart and soul of Tredegar and of the valleys.
"When fully restored it has the potential to be one of our finest attractions - linking up with the heritage features in Tredegar town centre including our famous clock."
The public is being given the first glimpse inside a £5.6m restoration project to save an historic south Wales mansion from dereliction.