MillerCoors, the combined US beer interests of SABMiller and Molson Coors, launches today in what Graham Mackay, chief executive of SABMiller, described as "an historic day in the American beer business".
The new company, with five board members each from the two partner groups, is likely to have a combined turnover in excess of US$7bn (£3.5bn) in the US market.
Management hopes it will be able to extract at least US$500m (£250m) in cost savings annually.
Molson's brands include Coors Light and Molson Dry; while SABMiller's best selling beers include Peroni Nastro Azzuro and Miller Genuine Draft.
Speaking as the new entity began its first steps, MillerCoors chairman Pete Coors said: "As a unified company with a world-class board and leadership team in place, MillerCoors will be able to create tremendous opportunities for innovations in products and services that will allow us to drive profitable growth."
SABMiller's Graham Mackay said the deal was not only a good one for shareholders in both groups "but for MillerCoors consumers, employees, distributors and business partners.
"Now that the transaction has closed and MillerCoors is a reality, the strong leadership team we have put in place is ready to execute and realize the tremendous potential of this great organisation."
MillerCoors begins trading as analysts anticipate a battle of wills elsewhere between US brewer Anheuser-Busch (A-B) and its potential suitor, InBev.
A-B has already rejected an InBev offer of US$46bn (£23bn), although observers expect European/Latino brewing giant to come back to the table with an improved offer.