Brentnall played on the wing for New South Wales in the inaugural 1980 State of Origin game and scored the first try in Origin history. Later that year played in Canterbury-Bankstown's Grand Final-winning side. In 1982, he became Canterbury's first Rothmans Medal winner and also toured with that year's undefeated Kangaroos, "The Invincibles", playing all 6 tests on tour, including Australia's first ever test against Papua New Guinea which was played immediately prior to the tour.[5]
In 2004, Greg was named as a reserve in the Berries to Bulldogs 70 Year Team of Champions. On 1 April 2007, he was inducted into the Bulldogs Ring of Champions.[6]
Post-playing
Brentnall retired from playing after the 1983 NSWRL season at the age of just 26 to take up the position of RL development officer in the Riverina for thirteen years.
[7]
Brentnall joined the newly formed Melbourne Storm as assistant coach under former team-mate Chris Anderson. Later appointed football development manager, he continues to work there and the club have honoured him by naming the young achiever (under 18 player) trophy after him. He was also appointed chairman of the Victorian Rugby League.[8]
