Originally from Victoria, and educated at Xavier College, Johnson played under-18s football for Oakleigh Chargers in the TAC Cup, serving as captain in his final season.

His poise and strong one on one contesting were the standout features of his game throughout 2011 and he also improved his rebound and attacking play as the season wore on.

Johnson went on to be a key player in the Sydney Swans' AFL Premiership–winning side, playing every game in the regular season and the finals series.

His run of injuries began when he tore his left anterior cruciate ligament (ACL) in Sydney's 2013 pre-season match against the Gold Coast Suns.

It was not until September 2016 that Johnson was even able to return to running laps after this second injury, as it took a further three reconstruction surgeries between 2014 and 2016 before his knee successfully healed due to various complications and infections.