The company, which is owned by the Bulloch family, bought Milnathort-based Forth after a proposed sale to a firm from England fell through several months ago.
It comes 20 years after Glasgow-based IM, then trading as William Morton, first tried to buy Forth.
The deal, financed through the firm's facility with Clydesdale Bank, will give the enlarged entity a turnover of about £75 million and a workforce of more than 200.
It combines IM's 2000 customers and a similar number of Forth accounts, with the merged business benefiting from a lack of crossover in their dealings. Accounts held by the merged entity include Gleneagles, G1 Group and Caledonian Heritable, owner of The Dome in Edinburgh.
IM managing director Stephen Russell said the acquisition would double the volume of wine it sells.
Noting its wine list was likely to expand from 1200 to 1600, he said it will strengthen its hand in talks with suppliers, allow it to reach a broader geographical base, and enhance its distribution efficiency.
Mr Russell said: "Morton at the moment is bursting at the seams.
"We go to Perth, Dundee, Aberdeen, Inverness, but we go from the south side of Glasgow. Once we get the logistics right between the two companies - and we won't do anything in the next three months, this is the busiest quarter of the year - we will try and service companies from the best geographical point.
"I suspect we will be increasing the number of jobs in Milnathort to cope with the volume that we are going to give them."
The acquisition signals the chance for IM to expand into England, where Forth generates £6m (22%) of turnover compared with IM's £1m.
Mr Russell, who presided over William Morton's merger with Inverarity Vaults in 2011, refused to rule out further acquisitions, hinting its next deal may be south of the Border.
He said: "Never say never. I am not actively scrolling through a list. If you look at it logically, we are six million people in Scotland, [while] England's got 60 million people. All the English wine merchants have no problem piling into Scotland and trying to take the Scottish business.
"It is not that tomorrow I am going to conquer the world, but I would say there is every likelihood that if we wanted to grow the business again maybe the next acquisition would be a company down south."
Forth's senior management team, George Thomson, Ian Cumming, Ewen Cameron and Alan Cramond, who led its buyout from Matthew Clark with private investors in 2010, are thought to have made a "premium" on the sale of their collective 30% stake, though an exact amount has not been disclosed.
Mr Cumming and Mr Cramond will move across to IM in the respective roles of commercial and finance director, with the other two directors pursuing other interests.
Mr Russell said the acquisition has brought together experienced and talented drinks trade executives on both sides, noting that IM sales director Donald Campbell had spent 20 years working at Forth.
Asked about current trading in the licensed trade, Mr Russell, who has worked for IM for 38 years, said the company is likely to report its best ever results for the year to September when the accounts become available.
Highlighting the continuing attraction of Scotland as a tourism destination, and the buoyancy of the whisky trade, he said: "The guys that are working at their businesses, they are doing OK. The people that fold their arms and expect it come to them, they are struggling. And there are plenty of good guys out there."
Mr Russell added: "We as a business have had some good wins in the past year."
The acquisition is the latest in a series of takeovers which have consolidated the Scottish drinks wholesale sector in recent years.
Forth acquired JA Glass of Dysart, Fife in early 2012, before IM snapped up boutique Glasgow drinks company LA Wholesale last year.
West Lothian-based Wine Importers has acquired a series of Scottish-based merchants since becoming part of Sir David Murray's business group in 2006, including Cockburn's of Leith, Harris Fine Wines and TM Robertson.
More recently the owner of Tennent's Lager, Dublin-based C&C Group, took a 50% stake in Irvine-based Wallaces Express, while Matthew Clark recently became the major distributor for Carling lager owner Molson Coors in the Scottish free trade.