Brought up on the family cattle station on the fertile plains of Northern New South Wales Brien Cobcroft was an all-round horseman who excelled in half a dozen different equestrian disciplines. After leaving school he worked as a jackaroo and used those the skills learn there to became national champion in the uniquely Australian pursuit of campdrafting (cattle herding on horseback). Cobcroft, along with his brother and business partner John, was also part of the successful Willow Tree polo team before turning his attention to eventing in the early sixties. Just two years after he took up the sport he was chosen for the 1964 Olympic Games where he finished 13th after excelling in the cross country phase. Four years later in Mexico he achieved the same individual placing and won a team bronze medal alongside Bill Roycroft and Wayne Roycroft. This proved to be his last Olympic appearance although he continued to compete for many years and eventually returned to his ranching roots in the sport of cutting, an American cousin of campdrafting.
The Cobcroft brothers farmed 9000 ha on the Liverpool plains and regularly produced prize winning cattle including three consecutive grand champions at the Royal Queensland Agricultural Show.