He played junior rugby league for Ipswich Brothers. At age 10, Keary moved to Sydney with his family. In Sydney, Keary played junior rugby league for the Kellyville Bushrangers and later Hills District Bulls and attended Oakhill College in Castle Hill. In 2010, Keary represented the Australian Schoolboys rugby union team. That same year he represented the New South Wales Combined Independent Schools (NSWCIS) rugby league team at the Under 18’s National Championships. In 2011, Keary returned to Queensland, moving to the Gold Coast where he attended Griffith University and played a season with the Burleigh Bears, playing for their FOGS Colts and Queensland Cup teams. At the end of the year he won Burleigh’s FOGS Colts Best Back Award. 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. Keary played for the Rabbitohs’ NYC team in 2012, captaining the team and playing 24 games, in which he scored 20 tries. At the end of season, he was named on the bench in the 2012 NYC Team of the Year and won South Sydney’s NYC Best and Fairest award.

In January 2013, Keary was a member of the Queensland Junior Emerging Origin squad. In Round 12 of the 2013 NRL season, Keary made his NRL debut for the South Sydney Rabbitohs against the Newcastle Knights off the interchange bench in the Rabbitohs 25-18 win at ANZ Stadium. In Round 20 against the Gold Coast Titans, Keary scored his first and second tries NRL career tries off the interchange bench in the Rabbitohs 32-4 win at Cbus Super Stadium. On August 22, 2013, Keary extended his contract with the Rabbitohs to the end of the 2015 season. Keary finished his debut year in the NRL with him playing in 10 matches and scoring 2 tries for the South Sydney Rabbitohs in the 2013 NRL season.

In February, Keary was selected for the Rabbitohs inaugural 2014 Auckland Nines squad. Keary was preparing his breakout year but it was ruined early due to a torn pectoral muscle injury in the Nines. Keary made his return to the Rabbitohs team in Round 17 against the Titans, scoring a try in the Rabbitohs 14-10 shock loss. On 5 October, in the 2014 NRL Grand Final against the Canterbury-Bankstown Bulldogs, Keary played at five-eighth in the 30-6 victory. Keary finished the season with 3 tries from 12 matches.

Keary’s State of Origin eligibility was a point of discussion within the media due to his being born and raised, for the first 10 years of his life, in Ipswich, Queensland. He spent the following eight years in Sydney, before returning to Queensland for another two years. In 2014, Keary wrote to then NRL CEO Dave Smith requesting that he be considered a Queenslander for future representative opportunities. The request was denied and the 2012 eligibility rules were upheld.

On 27 January, Keary re-signed with the Rabbitohs on a 1-year contract. On 23 February, he played for the Rabbitohs in the 2015 World Club Challenge match against 2014 Super League Grand Final premiers St. Helens, playing at five-eighth and scoring a try in the Rabbitohs’ 39-0 win at Langtree Park. In April, he was selected for the New South Wales City team for the annual City vs Country Origin match, but withdrew due to a foot injury. He finished off the 2015 season having played in 24 matches and scoring 4 tries for the Rabbitohs.

At the start of the year, Keary, who was off contract at season’s end, was linked to sign with the Sydney Roosters, the St George Illawarra Dragons who offered him a contract worth $1.2 million over 3-years, or even a shift to Rugby Union for 2017. On 21 June, Keary announced that he signed a 2-year contract with the Roosters starting in 2017. In Round 21 against the Canberra Raiders, Keary would play his last match for the Rabbitohs after he suffered a season ending hamstring injury in the woeful 54-4 loss at ANZ Stadium. Keary finished his last season with the Rabbitohs with him playing in 17 matches and scoring 3 tries in the 2016 NRL season.

Luke Keary married high school sweetheart Amy Bugeja on 14 October 2017.

In 2018, Keary suffered a broken jaw at pre season training and missed over a month of training and trial games.

Keary received the Clive Churchill medal in the 2018 Grand Final where his team the Roosters beat the Melbourne Storm 21-6.

Keary made his international debut for Australia in 2018 against New Zealand. In 2020 he was selected to make his State of Origin debut for New South Wales in Game I.

Keary started the 2019 NRL season in good form as the club won 7 of their first 9 games. Keary’s good form meant that he was in line to be selected for New South Wales in the 2019 State of Origin series. In Round 11 against Newcastle, Keary was taken from the field with concussion and was subsequently ruled out for 6 weeks.

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. It was Keary’s third premiership victory as a player.

In round 9 of the 2020 NRL season, Keary scored two tries as the Sydney Roosters defeated North Queensland 42-16 at Queensland Country Bank Stadium. In August 2020, Keary extended his contract with the Sydney Roosters until the end of the 2024 season. In round 18 against Newcastle, he scored two tries as the Sydney Roosters won the match 42-12 at the Sydney Cricket Ground.