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, he played junior rugby league for the Hills District Bulls and attended Oakhill College in Castle Hill.
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.