 Brennan was signed as a marquee player by the Bulldogs ahead of the inaugural AFL Women's season in 2017, and was their leading goalkicker in 2019.Brennan grew up on a large family property in Loganholme, a suburb 29 kilometres south-east of Brisbane but also spent periods of her youth living in England and Dubai.She attended high school at Our Lady's College in the suburb of Annerley.