Cahill first turned professional in 2013, aged 17, after winning the European Under 21 Championships, but returned to amateur status in 2017.

O'Sullivan was the second former world number one Cahill had defeated in the 2018–19 snooker season, after his victory over Mark Selby at the 2018 UK Championship.

Cahill qualified for a new two-year tour card as the second highest ranked player on the one year list who was not in the top 64 in the world after the 2018–19 season.

At 17, he defeated Joseph McLaren and Ross Muir 4–3, Elliot Slessor 4–1 and Darryl Hill 5–1 in the semi-final to play sixteen year old Ashley Carty in the final.

Having won the event, Cahill gained a two-year tour card for the 2013–14 and 2014–15 snooker seasons, allowing him to play in all professional tournaments.

[9][10] He played in the main stages in the 2013 UK Championship and the 2013 Welsh Open, but did not progress past the first round in either competition, losing to Joe Perry 6–3, and Ken Doherty 4–0.

He won comfortably in the first round, beating veteran Mark King 6–0 and then saw off former Welsh Open finalist Andrew Higginson 6–4.

[13] Cahill faced world number three Ding Junhui, a player who had won five ranking titles the previous season, and raced into a 5–1 lead.

[14] In the last 16 of the championships, Cahill commented he couldn't "believe how badly [he] played" after his tournament ended with a 2–6 loss to Mark Davis.

[15] Cahill had a poor end to the season as he lost seven out of eight matches after this which would have relegated him from the tour as he finished 85th in the world rankings.

Appearing at the 2016 Welsh Open, he won his first round match for the first time, defeating Xiao Guodong 4–1, but then lost 1–4 to Mark Davis.

[20] Cahill progressed to the third round of the 2016 Paul Hunter Classic by eliminating Kevin Vandevoort 4–0 and Ryan Day 4–2.

He did, however, defeat Robbie Williams and Noppon Saengkham before losing to Ken Doherty in the third round of the non-ranking one-frame Snooker Shoot-Out event.

[24] Having finished the season with a world ranking of 106, Cahill required a successful run in 2017 Q School to remain on the tour.

Cahill defeated Judge 10–6 to become the first amateur player to play at the main stages of the world championships at the Crucible Theatre.

"[40] The match was referred to as the "biggest shock in Crucible history" by the BBC, with master of ceremonies Rob Walker introducing him as "the giant killer" for having defeated two world number ones as an amateur.

[50] [51] Despite having reached the second round of the world championships, Cahill started the following season with no ranking points, not having achieved the results as a professional player.