He won the Belgian U18 championship in 2018.[2] In August 2018 he played in a professional ranking tournament for the first time, and beat Adam Stefanow in the first round of the 2018 Paul Hunter Classic.[3]
In October 2018, when he was thirteen years old, he won the World Open Under-16 Snooker Championships, becoming the first male Belgian snooker world champion (Wendy Jans is a multiple winner of the senior women's world championship).[3]
In January 2019, he defeated Michael White, then ranked #36 in the world, at a snooker tournament in Bruges.[4]
In July 2020 he defeated James Cahill in the first round of the World Championship qualifiers, becoming the youngest player ever to win a match in the World Championships. This record was broken by Liam Davies, who was two days younger than Mertens when he won his first match at the 2022 World Snooker Championship qualifiers.[7]