The millionaires spent only 10 minutes viewing Frognal House, a tea merchant's mansion in Troon, before deciding to buy it - complete with furniture and fittings.
The couple from Largs became the UK's biggest lottery winners in 2011 after winning £161m in a Euromillions draw.
Frognal has four main bedrooms overlooking grounds which include fountains, greenhouses and a market garden.
The mansion's previous owner, hotel and restaurant magnate Bill Costley, had lived there for seven years and spent £2m renovating it.
He said the Weirs had been looking at one of his commercial properties, Doonbrae House in Alloway, but found its five-acre estate too small.
He said: "They were in about 10 minutes and bought it and all the contents.
"I say it myself, but it is a very special house in a unique location and I am sure the Weirs will be very happy in it, as we were."
Last year, former STV cameraman Colin, 65 and Chris, 57, launched the Weir Charitable Trust, investing £5m to support good causes across Scotland.
They have also donated money to the SNP and Partick Thistle football club.