Harmon's professional career started when he was named in Canterbury's squad for the 2016 Mitre 10 Cup. He debuted in the first match of the season, a 43-3 victory over Auckland, and went on to make 7 appearances and score 3 tries during the campaign. Billy was an integral part of the side that lifted the Premiership title for the 8th time in 9 seasons and claimed the Ranfurly Shield.
Billy joined the Highlanders from the Crusaders at the start of 2021. After being named Highlanders Defender of the year, he was later selected for the Māori All Blacks to take on Manu Samoa in a two-test series.
The reliable openside flanker will again play for Canterbury during the Bunnings NPC before returning down south to hopefully continue his impressive form at Super Rugby level.