When Billy Hartung was 14 years old he set a Mornington Peninsula Junior Football League record for most goals kicked in a single game. His effort of 20 goals straight was done while playing for Red Hill under-14s against Dromana. Hartung progressed to the VSFL under 18 competition and played with the Dandenong Stingrays. Selected an All-Australian at under-18 level in 2013, Hartung attended the AFL Draft Combine in October 2013 when he ran a 16.6 beep test, breaking (future teammate) Bradley Hill’s record of 16.1.
Hawthorn selected Hartung with pick 24 in the 2013 AFL Draft. Hartung has great evasive skills and good foot skills on both sides of his body.
He started 2014 developing in the VFL playing for Hawthorn's reserve affiliate side Box Hill. Good form lead to his promotion and senior debut against St Kilda. He played five consecutive games between rounds 8 and 12, before an elbow injury sustained against Sydney in Round 12 meant that he missed a month of football, he returned to the VFL. He played in Box Hill's losing Grand final side.