This recent creation was developed in England by a small collective of enthusiasts from Birmingham who wanted to establish a Molosser breed which would be superiour to the English Mastiff in terms of agility and working ability, suited for use in personal protection, property guarding, police and military duties, as well as being a healthy and trainable family companion. By using the imported Rottweilers from German working lines and crossing them with the best English Mastiffs, Bullmastiffs and Neapolitan Mastiffs available in the United Kingdom, the breed creators established the foundation stock from which the Mastweiler was developed through careful selection based on health, stamina and intelligence, with little attention paid to the physical appearance. Once the temperament and working qualities were set, selection based on type, size and colouring resulted in a reasonably uniform breed, but due to the continuing use of pure Rottweilers and English Mastiffs as outcrosses, the Mastweiler population still shows some variation in terms of overall appearance.
Although no other breeds were officially used in the programme, a small percentage of Presa Canario, Great Dane and Irish Staffordshire Bull Terrier blood is also rumoured to had been employed early on, with some strains reflecting that influence more than others. It should be noted that not every cross of English Mastiff and Rottweiler is necessarily a Mastweiler, but this name is commonly used to label anything from 1st generation crosses between Rottweilers and English Mastiffs to any mongrel containing blood of these two breeds by those wishing to capitalize on the name recognition of the Mastweiler breed. Most fanciers of the Mastweiler breed insist that any unregistered crossdog that could be mistaken for this Molosser from Birmingham should be called a "Rottstiff" instead, which is a name long abandoned by the early developers in favour of the more popular English Mastweiler moniker.
The Mastweiler is a well-tempered and stable dog, said to be gentle with children and devoted to its master, but somewhat aloof with strangers and very protective of its territory and family, needing experienced ownership and early socialization. Dog-aggression is undesirable, even though those lines suspected of having some Presa Canario and ISBT ancestry tend to be more confrontational than the rest. This is a large, broad-shouldered and wide-chested Molosser, with a broad head, strong muzzle and reasonably loose skin. The tail and ears are left in their natural state, but specimens with docked tails can be found too, although such animals aren't as valued as unaltered dogs. Essentially a bandogge, the English Mastweiler shows some inconsistency in terms of physical type and depending on the actual bloodline, some dogs can be fairly heavy and tall, while others are shorter and have leaner bodies, appearing more "bully-esque" overall.
The coat is short, thick and flat, preferred in dark brindle colourings, but also allowed in solid fawn, wheaten, red and brown shades, although some black, black-n-tan, as well as dogs with white markings can be encountered on occasion. Average height is around 27 inches, but taller, as well as smaller examples exist.