NEVER work for a family firm, they say. Leo Kiely has merged two. Is he mad? “No,” he laughs, “but I could write a book about it.”

Kiely, a pudgy Irish-American who collects Harley-Davidson motorbikes, looks like he is enjoying the ride. He has headed Molson Coors, the world’s fifth-biggest brewer, since February when the two famous old beer firms lashed themselves together to create a giant with sales of $6 billion (£3.3 billion).

Since then, however, it has been rough roads and hold on to your pints, boys. Earnings have fallen, legal action has been launched by shareholders (unhappy at what wasn’t revealed before the merger), and stock sales by Kiely and a colleague have come under scrutiny.

The tremors are already hitting the shores of Britain. Molson Coors, based in North America, runs Coors Brewing Ltd, owner of Britain’s No1 lager brand Carling, as well as Grolsch and Coors Light. Kiely has recently put in a new boss and is looking for cost savings in a market where competition is becoming increasingly fierce.

But if any of this is causing him anxiety, you would be hard-pushed to spot it. Holding court on a sofa in a busy London hotel foyer, Kiely banters away happily, using his gravelly laugh for punctuation.

He loves Britain, he says, because it’s got the best beer-drinking experience in the world. “America and Canada can be exciting, but if you’re a beer guy, Britain is what beer is all about.”

Diplomacy, as you would expect from an executive answering to two family factions, is a Kiely strong suit. Add that to his comfy beer-belly, lived-in face and neat line in homely bizspeak — “New sheriff in town, he can do whatever he wants,” he shrugs, when I query succession — and 58-year-old Kiely looks like the kind of brewery boss who would be happiest pulling his own pints.

His CV tells another story, however. Born in Massachusetts and business-schooled at Wharton, he had already done tough stints at Procter & Gamble, Wilson (sporting goods) and Frito-Lay (corn chips), before joining Coors in 1992.

His only link with the beer business was his grandfather’s pub in his home town of Marblehead, north of Boston. If granddad was anything like grandson, that would have been beer served with a big smile on top and a baseball bat under the bar.

Kiely is going to have to be tough if he is going to sort out the problems that are assailing Molson Coors. Profit has nosedived since the merger and at world No5 (a long way behind Inbev, SAB Miller. Anheuser Busch and Heineken) it still needs more scale to compete globally.

That was why Denver-based Coors and Montreal-based Molson, both public companies but controlled by founding families, got together in the first place. The marriage also provided protection against the prowling European giants.

Now Kiely has to make it work. The group is split into four divisions — America, Canada, Europe and Asia, and Brazil — but a dispute over tax at its Brazilian subsidiary, plus fierce competition everywhere, is now worrying analysts.

Many remain unconvinced that the merger will provide effective synergies, especially as Molson Coors keeps a dual base structure — chairman Eric Molson in Canada and Kiely and vice-chairman Peter Coors in America.

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