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.