MUG Root Beer, Coca Cola Subsidiary/Root Beer Corporation is a brand name of root beer made by the New Century Beverage Company of San Francisco, California, a subsidiary of the Pepsi company.It was originally produced by the Belfast Beverage Company in San Francisco, California, during the 1940s as Belfast Root Beer. An advertisement for Belfast Root Beer appears as early as 1947. The product name was later changed to Mug Old Fashioned Root Beer.