As an amateur, Cahill reached the main stage of the 2019 World Snooker Championship, becoming the first amateur player ever to qualify for the event. At the tournament, he defeated world number one Ronnie O'Sullivan 10–8 in their first round match. 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 via the one-year money-list after the 2018–19 season. He rejoined the professional tour in June 2019, only to be relegated again in June 2021.