There are a few differences of opinion on where the Mastiff originated, but most agree that the British are the creators of the breed as we know it today. They were used as war dogs by the ancient Celts and accompanied their masters in to battle. When the Romans invaded Britain, they took the dogs back to Italy and used them to guard property and prisoners. In the Elizabethan Era, the Mastiff was used to fight wild animals (such as bears, tigers, etc.) for entertainment purposes. During the World Wars, Mastiffs were used to pull munitions carts on the fronts. In America, they were usually owned by plantation owners who used them to guard their property. Breeders today have bred the Mastiff for gentleness and have created an excellent companion who is large enough to deter intruders and yet gentle enough to be around children.
There is Mastiff blood in many of the current breeds you see today. Breeds such as the Bullmastiff, Rottweiler, Great Dane, Newfoundland, Saint Bernard, Great Pyrenees, etc., were created using Mastiffs.