Mastiff was a steep grey basic wedge shaped robot armed with a flipping arm similar to Mad Dog. On its wedge, it featured the image of an angry dog, like that of Bulldog Breed. Mastiff was capable of running on its back if it was flipped over and had side spikes to stop it from being flipped onto its side, but had a suspect ground clearance of 1cm. Mastiff entered the Second World Championship in Robot Wars Extreme: Series 1 and Extreme Warriors: Season 1, but lost in the First Round.
Mastiff faced UK Champions Chaos 2, US robot Manta and German robot Ansgar in its only battle, which was also broadcast as part of German Robot Wars. It was immediately targeted by Chaos 2 and was flipped over several times, however no damage was caused internally. The pit was activated and Chaos 2 pushed Mastiff near the edge. However Chaos 2 tried to flip Mastiff into the pit, but only flipped it away and drove itself down instead. Mastiff didn't have time to recover as it was quickly pushed down the pit by Manta, going out of the World Championship.