He competed at the 2022 Commonwealth Games for Northern Ireland, where he won the silver medal in the men's 1500 metre freestyle event.

[7][8] He won his first senior international medal at the 2022 Commonwealth Games, where he placed second in the men's 1500 metre freestyle event and competed for Northern Ireland.

Wiffen won gold medals in the 400, 800 and 1500 metre freestyle events at the 2023 European Championships (25m) in Otopeni, where he also set a new world record in the 800 metre freestyle short course with a time of 7:20.46 and became the first Irish swimmer to win a European short-course gold medal.

He qualified in first place for the 800m freestyle final and on 30 July, he won the gold medal with a new Olympic record time of 7:38.19.

He was the second Irish swimming medalist at the 2024 games and the first individual Olympic Gold medal winner from Northern Ireland since 1972.