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.
1955 ad for Belfast Old Fashioned Mug Root Beer
An advertisement for Belfast Old Fashioned Mug Root Beer appears as early as 1952. In the late 1960s, Sugar Free Mug (now Diet Mug Root Beer) was introduced. Mug Cream Soda and Diet Mug Cream Soda were later introduced, but they are not as widely available. Mug was purchased by Pepsi in 1986. Mug Root Beer is owned by Pepsi, but it is manufactured by independent bottlers under the authority of New Century Beverage Company of San Francisco, California. One can validate this information by looking on the side of a Mug Root Beer can.
The Mug can features the company's mascot, a bulldog named Dog, holding a mug of root beer. On the Mug Root Beer website there is a free online comic to view the dog's adventures.