The National Trust for Scotland said the funding was among the largest anonymous donations it had ever received and would go towards the Mackintosh Illuminated scheme.
It is centred around raising funds to support two publicly accessible Mackintosh sites – the Hill House in Helensburgh and the Mackintosh Tearooms on Sauchiehall Street in Glasgow.
The overall project is estimated to cost £20m, with the National Trust seeking to cover £14m of that total.
The project also plans to deepen public understanding of Macintosh’s wife, Margaret Macdonald Mackintosh, whom he credited for much of his success and who was a skilled artist in her own right across a range of disciplines.
The Mackintoshes were married in St Augustine’s Episcopal Church in Dumbarton High Street and lived for a time at Dunglass Castle in Bowling.
It is hoped the project will show how the couple’s artistic contribution, creative partnership and influence helped shape Glasgow’s style.
The charity are hoping to accomplish this by 2028, which will mark 100 years since Mackintosh’s death.
Phil Long OBE, the chief executive at the National Trust for Scotland, said the donation was hugely appreciated.
He added: “Building on the support of the National Lottery Heritage Fund, we have ambitious plans to shine a light on the work of Mackintosh and Macdonald, to continue our pioneering conservation work at the stunning Hill House and to bring all of this to people in Scotland and beyond.”
The 120-year-old Helensburgh building is considered to be Mackintosh’s most complete example of a domestic home.
It was placed inside a chainmail box several years ago in an attempt to protect its saturated walls from further damage and allow them to gradually dry out.
Work at the tearooms will “strengthen interpretation, visitor engagement and future programming” at the site.
The tourist attraction is housed in Catherine Cranston’s original building, where she commissioned Mackintosh and Macdonald to design their famous interiors.
The National Trust claim this work will help position the Tearooms as a key cultural destination within the city centre.
Ali MacLeod, the head of fundraising at the National Trust for Scotland, called the donation an “extraordinary leadership gift which shines a powerful light on the lasting importance of Mackintosh and Macdonald’s creative legacy”.
She added: “It is through the generosity of donors like this that we can safeguard these remarkable places and stories, while inspiring the designers, artists and makers of today and tomorrow.”
Top of page picture: Margaret and Charles Rennie Mackintosh, who were married in Dumbarton and lived on the banks of the Clyde at Bowling.