Bullmastiffs are a large working breed originating in Britain in the mid 1800s as estate guardians to ward off poachers. They were bred by gamekeepers for strength, size and speed using a cross of the tough, heavy and aggressive Bulldog of the 19th century with the large, strong, less aggressive Mastiff. As a result, the Bullmastiff is known as the Gamekeeper's Night Dog. The preferred color, by gamekeepers, was brindle, as this color works as a more effective camouflage, especially at night.
The Bullmastiff breed was deemed pure by the English Kennel Club in 1924, with a genetic background of approximately 60% Mastiff and 40% Bulldog. In 1934, the AKC recognized the Bullmastiff. The first standard for the breed was approved in 1935. The standard has undergone several revisions since then; the most current version is available on the AKC website. In 1928, the diamond mining company De Beers imported Bullmastiffs to South Africa to guard the mines.