The deal includes John R Weir's Mercedes-Benz car dealerships in Grangemouth, Perth, Dundee, Aberdeen and Inverness; its Kia and Chrysler Jeep dealerships in Perth; Smart dealerships in Aberdeen, Inverness and Perth; and Mercedes-Benz commercial vehicle dealership in Aberdeen.
The outlets will continue to operate under the name of Grangemouth-based John R Weir, and its 321 employees will be retained.
John Weir, managing director and principal shareholder of the eponymous group, turned 65 in July.
He said: "The board of John R Weir Group reviewed a range of options for the group going forward and determined that the sale of the company was best for the business, its customers and its staff.
"I am personally delighted that we have been able to attract a buyer of the calibre and size of Arnold Clark, who I am confident will grow the business further."
The deal will increase Glasgow-based Arnold Clark's presence in top-end cars by giving it its first Mercedes-Benz dealerships. It already sells BMWs. Arnold Clark managing director Eddie Hawthorne said: "The John R Weir Group is synonymous with high standards of quality and customer service, and we are proud to be welcoming the whole John R Weir Group team into the Arnold Clark family.
"We are also proud to be adding Mercedes-Benz to the portfolio of manufacturers we represent and we are confident that together we will make a great success of this new acquisition."
Last month, John R Weir reported a 41.3% fall in pre-tax profit to £249,000 for the 12 months to October 31, 2011.
This was despite an increase in turnover from £91.5m to £92.1m.
This compares to turnover of £2.3bn at Arnold Clark, Europe's largest privately owned car dealer, when it last published accounts for the 2010 calendar year.
Car sales have been under pressure in recent years, especially since the Labour government's car scrappage scheme was axed, and value added tax increased in 2010 and 2011.
There have been signs of hope with recent figures from the Scottish Motor Trade Association showing the number of new registrations in Scotland rose by more than 24% to 12,036 in July this year, compared with 9701 in July 2011.
The industry has attributed this to drivers seeking more fuel-efficient models to counter price rises.