Born in Ipswich, Queensland and raised in the suburb of Raceview, Keary attended St. Mary's Primary School in Ipswich and 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 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 New South Wales 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.