[1][2] Byrnes played junior football for his local club, St Paul's McKinnon JFC.

[8][9] Byrnes debuted in the Saints' 23 point victory over the Adelaide Crows in the 7th round of the 2020 AFL season, becoming the 1616th player to represent St Kilda.

[12] He signed a one-year contract extension with St Kilda on 7 February 2021, keeping him at the club until at least 2022.

[14][15] Byrnes was called up to the senior side in round seven for just his second AFL game and collected 18 disposals and nine marks.

He was also impressive against top four side Brisbane in Round 17, collecting 15 disposals, had seven score involvements and kicked two goals in the Saints' upset win.