[5] That same year he represented the New South Wales Combined Independent Schools (NSWCIS) rugby league team at the Under 18s National Championships.

[8] Growing up, Keary supported the Brisbane Broncos, idolising Allan Langer who also hails from Ipswich, and stated his ambitions of one day representing them before signing with the South Sydney Rabbitohs ahead of the 2012 NRL season.

[13] In Round 20 against the Gold Coast Titans, Keary scored his first and second tries NRL career tries off the interchange bench in South Sydney's 32–4 win at Cbus Super Stadium.

Keary was preparing his breakout year but it was ruined early due to a torn pectoral muscle injury in the Nines.

[16][17] Keary made his return to the South Sydney team in Round 17 against the Gold Coast Titans, scoring a try in the clubs 14-10 shock loss.

[29][30] In Round 21 against the Canberra Raiders, Keary would play his last match for the South Sydney Rabbitohs after he suffered a season ending hamstring injury in the woeful 54–4 loss at ANZ Stadium.

In Round 1 of the 2017 NRL season, Keary made his club debut for Sydney Roosters against the Gold Coast Titans, having a successful match by scoring 2 tries in the 32–18 win at Cbus Super Stadium.

[36] Keary received the Clive Churchill medal in the 2018 Grand Final where his team the Sydney Roosters beat the Melbourne Storm 21–6.

He capped a stellar season by earning selection in the Australian Kangaroos team as starting five-eighth for Test matches against New Zealand and Tonga.

Keary's good form meant that he was in line to be selected for New South Wales in the 2019 State of Origin series.

[39] Keary played at five-eighth for the Sydney Roosters in the 2019 NRL Grand Final in which the club won their second consecutive premiership defeating Canberra 14–8 in the decider.

[46] In round 8, Keary kicked the winning field goal in the Sydney Roosters 27–26 victory over St. George Illawarra in the ANZAC Day game.

[48] Keary played 26 matches for the Sydney Roosters in the 2023 NRL season as the club finished 7th on the table and qualified for the finals.

[50][51] On 17 June, Keary announced he had changed his mind about retirement signing a deal to join the Catalans Dragons in the Super League starting in 2025.

[52][53] Keary played a total of 26 games for the Sydney Roosters in the 2024 NRL season as they finished third and qualified for the finals.

In 2014, Keary wrote to then NRL CEO Dave Smith requesting that he be considered a Queenslander for future representative opportunities.

[59] On 16 October 2022, Keary made his debut for Ireland at the 2021 Rugby League World Cup and earned man of the match honours in their 48–2 victory over Jamaica.