Taylah Egbers, visitor services supervisor at the National Trust for Scotland (NTS) property, stumbled across the 30 slides while doing research online and was intrigued by the images, which include views of the interior and exterior of the house in Helensburgh.
The pictures, bought from a seller in America, were taken by an architect who enjoyed travelling and visited the Hill House in the 1970s.
Conservationists said the images will help them create a visual timeline of decorative alterations made by previous owners, such as the ceiling in the drawing room, which was plum coloured for several decades.
Miss Egbers told the PA news agency: “I stumbled across the slides online and thought ‘those are brilliant’. From the advert I saw they were not really images that I’d seen before and I thought those will be great for the house to have and also for the conservation project.”
The seller had bought the slides from an architect’s estate sale.
The architect’s wife had told the seller that in the 1970s her husband, who enjoyed photography, spent lots of time travelling around famous places, including Hill House.
Miss Egbers added: “We are so lucky he took the photos. It’s not very often that you get a set of 30 slides and it’s nice because they are so personal, you do get a completely different view of the property than you would if it had been a professional, because they are just somebody’s holiday snaps of the property.
“And that’s the reason I was keen to get them because I knew that they’d be angles that aren’t necessarily repeated by professional people who come to the house, so I knew that would be a different aspect to see them.
“That’s why I was really keen to see them because it had that kind of personal view of the house.”
Miss Egberts said it was “very exciting” to find the slides, which were bought for around £30 in the eBay sale.
The Hill House in Argyll and Bute was designed and built by Mackintosh for publisher Walter Blackie between 1902 and 1904.
The slides show how one alteration brought in by the Blackie family in 1912 was still in place in the mid 1970s.
In 1912 the Blackie family asked Mackintosh to return to the property to alter the drawing room ceiling from white to dark plum and it remained that colour until NTS took over the care of the property in 1982 and restored the rooms to how they looked in 1904.
Miss Egbers said the photos showed that fireplaces in the house were still being used.
She said: “I thought it was quite beautiful to see the drawing room with the dark ceiling, which is also quite unusual to see, along with the fireplace being on.
“I thought that was really, really interesting, I’d not seen a photo with the fireplace on before so that was my favourite one.”
The house was sold to Thomas Campbell Lawson in 1953 and was then cared for by the Royal Incorporation of Architects in Scotland from 1971 until NTS took it over.
There are also slides of rooms such as the white bedroom, showing how little it has changed over the years, the library and hallway as well as several exterior shots.
Miss Egbers said the slides will be scanned and uploaded to the archive to help piece together more about the story of the house.
The house is currently protected by a giant “box” built to protect its saturated walls from further damage from the elements.