[3] Brennan started out in racing at the yard of trainer Jim Bolger with the intention of being a flat jockey but decided to switch to jumps on account of his weight.
[4] In the 2004/05 season Brennan became champion conditional jockey and achieved his first Grade 1 success with Ashley Brook in the Maghull Novices' Chase at Aintree in April.
[3][5] His first win at the Cheltenham Festival came in 2006, when he rode 40/1 outsider Shamayoun to victory in the Fred Winter Juvenile Novices' Handicap Hurdle.
[5] In November 2008 Brennan had his first ride on Imperial Commander for Nigel Twiston-Davies, winning the Paddy Power Gold Cup at Cheltenham.
[9] In April 2015, Brennan achieved his best placing in the Grand National, when 25/1 chance Saint Are, trained by Tom George, came second to Many Clouds.
[10] In October 2015 Brennan rode the Colin Tizzard-trained Cue Card for the first time, winning the Charlie Hall Chase at Wetherby.