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 was born in Brisbane, Queensland[1] and raised on a large family property in Loganholme, a southeastern suburb 29 kilometres the city.

[2] Her first game of competitive football came that same year when she filled in for her older brother's under eights side, where she kicked seven goals on debut.

[3] Brennan continued to play with the Logan Cobras boys through to girls eligibility age cut-off at 15, including in one season where she won a club best and fairest award at under 14s level.

She moved to Melbourne ahead of the 2013 season at the age of 20 and began playing for the Darebin Falcons in the Victorian Women's Football League (VWFL).

[11] In July 2016, Brennan signed with the Western Bulldogs as one of their two marquee players ahead of the AFL Women's inaugural season.

[23] On 23 March 2018, Brennan made a gender discrimination complaint against the AFL to the Australian Human Rights Commission over the two-match suspension which ultimately prevented her from playing in the Grand Final.

[33] Brennan achieved selection in Champion Data's 2021 AFLW All-Star stats team, after leading the league for average score involvements in the 2021 AFL Women's season, totalling 4.8 a game.

[34] In the 2021 AFL Women's season, Brennan was awarded with her maiden All-Australian blazer, named on the half forward position.