Cahill played the season as an amateur player, but allowed into the main stage of events through qualifying. Cahill defeated Liang Wenbo in qualifying to reach the 2018 World Open mainstage, but was defeated 4–5 by Andrew Higginson. At the dedicated Amateur qualifying event for the 2018 Paul Hunter Classic, Cahill defeated Conor Caniff 4–1 and Charlie Walters 4–2 to reach the event. However, Cahill lost in the first round once again to Niu Zhuang. Competing at the 2018 UK Championship as an amateur, Cahill defeated world number one Mark Selby 6–3 in the first round, allowing Selby just 79 points in the first three frames. After the victory, Cahill commented "the standard wasn't amazing. But to beat the world number one, such a great player, is brilliant." He then played Sunny Akani in the second round, losing 5–6.