Brennan was born and raised in Darwin in the Northern Territory, where he played rugby league[1] and Australian football at junior levels.

Brennan's unique ball skills and agility displayed his indigenous heritage with a liking to Daryl White.

Coach Leigh Matthews, switched him to the forward line in mid-2007, and against West Coast Eagles in Round 14, Brennan appeared to come of age, leading the charge with four goals.

Used predominantly as an impact player in the midfield, Brennan proved himself to be a capable clearance winner and played a decisive hand in several close victories.

Brennan capped off his 2008 season with a top 5 placing in the 2008 Brisbane Lions best and fairest and selection in the Australian International Rules squad.