Youth Mertens was born in
Amsterdam. He played for
SDW,
DWS and
AFC before joining the
FC Utrecht Youth Academy in 2008. Two years later he had to leave and decided to return to AFC. His return was not for long and after one season he moved to
SV Argon. In the summer of 2013 he made the move to
FC Den Bosch, where he played with the A1 (Under 19).
FC Volendam In the preparation of the 2014–15 season, Mertens joined
FC Volendam on an amateur basis. That season, he played 15 matches in the second team in which he scored 4 times. He also made it to the match selection of the first team two times, but didn't make his debut. On 30 March 2015, he signed his first professional contract with FC Volendam, until mid 2017. In the summer of 2015, Mertens joined the first team. On 7 August 2015, he made his debut for FC Volendam in the league match against
NAC Breda (4–1 win). He replaced
Rafik El Hamdi after 79 minutes. On 18 September 2015, he scored his first goal for FC Volendam, the opening goal in a 2–0 win against
Helmond Sport. In February 2016, Mertens extended his contract at FC Volendam until mid 2018. In the preparation of the 2016–17 season, Mertens tore off his ACL and was unable to play for the rest of the season. On 29 April 2017, 10 months later, he made his comeback in a match of Jong FC Volendam against
FC Rijnvogels.
Telstar On 21 June 2018, Mertens signed a one-year contract with an option for an additional season with Eerste Divisie club
Telstar. He made 19 appearances in the
2018–19 season, scoring twice. However, after a disappointing start on the season, the club announced on 17 October 2019, that Mertens was one of three players that had left the club.
Fakel Voronezh In June 2024, Mertens joined
Russian Premier League side
Fakel Voronezh. He left Fakel at the conclusion of the 2024–25 season, following Fakel's relegation.
Return to Telstar In early July 2025, Mertens returned to
Telstar for a trial ahead of the club's first
Eredivisie season since 1978, having previously represented them in the 2018–19 season. During pre-season, he addressed public questions about his spell in Russia following the country's
2022 Russian invasion of Ukraine, saying that he had weighed the decision carefully and that everyday life and league matches had continued as normal while he was there. On 8 August, he signed a one-year contract with an option for an additional season, becoming Telstar’s 18th contracted professional player in accordance with Eredivisie roster regulations. He made his second debut for the club on 10 August, replacing
Tyrese Noslin in the 76th minute of a 2–0 away defeat to
Ajax, and made his first start on 20 September in a 2–0 loss to
Groningen. ==Career statistics==