It was probably during his supposed reign that the Dumnonians made a push against Wessex in 658. Unfortunately, they were defeated at the Battle of Peonna (Penselwood, Wilts) and the Dumnonian border was set at the River Parrett. Cenwalh of Wessex was thus encouraged to try his luck further. In 661, he defeated the Dumnonians at the Battle of Posentesburh (Posbury, Devon). Having conquered the eastern portion of Brochmail's kingdom, Cenwalh introduced the Somerton-settlers, and Saxons began moving in as far west as Crediton. It was probably also during Brochmail's reign (682) that the West Saxons "drove the British as far as the Sea" (possibly around Bideford).

Brochmail only appears in the 17th century Llyfr Baglan. He is considered apocryphal.