The Great Swiss Mountain Dog is considered among the oldest of the 13 original Swiss canine breeds. Several theories exist with regard to the ancient origins of the 2 largest of the Swiss Sennenhunde breeds, the GSMD and the Bernese Mountain Dog.

The most popular theory states that these dogs are descendants of the Molossian, a large Mastiff type dog that accompanied the Roman legions on their conquest of vast areas of Central Europe in the 1st Century B.C.

Another hypothesis is that a large canine breed was brought to Europe by the Phoenicians about 1100 B.C. when they settled in Spain. Supposedly, these dogs later migrated eastward and influenced the development of large Mastiff type dogs such as the Spanish Mastiff, Great Pyrenees, Dogue de Bordeaux, Great Dane, Rottweiler and others as well as eventually the large Swiss breeds such as the Saint Bernard and the Great Swiss and Berner Mountain Dogs.

Yet another speculation assumes that a large breed was already in existence at the time of the Roman invasion of the alpine regions of Central Europe. The Roman dogs would have been crossed with these indigenous dogs. In Switzerland, these cross breedings eventually would have led to the development of the Saint Bernard and the two large Sennenhunde breeds, the Swissy and the Berner.