Born and raised in Newcastle, New South Wales, Australia, Brennan played his junior rugby league for the Western Suburbs Rosellas.

[2][3][4][5] In 2003, Brennan became the assistant coach for the Newcastle Knights Jersey Flegg Cup side.

In 2017, he once again coached the Panthers' NSW Cup side, winning his second premiership and the club's first NRL State Championship.

At the end of the season, he was set to be an assistant coach for New Zealand at the 2017 Rugby League World Cup before resigning to accept the Gold Coast Titans job.

On 14 July 2019, Brennan was terminated from his position as Gold Coast head coach after the club were sitting last on the table after only winning 4 out of 17 matches.