He is only the third male swimmer to be simultaneously short-course and long-course World Champion at 50 metre freestyle, after César Cielo and Florent Manaudou.

Proud holds two British national records, as well as being a double Commonwealth Games champion in both the 50 m butterfly (2014 and 2022) and 50 m freestyle (2014, 2018 and 2022) events.

[11][12] Proud won a silver medal in the 50 m butterfly -his strongest event- in the European Junior Championships, which put him on the map as the fastest British sprinter at the British Gas International Meet in 2013, winning a gold in the butterfly, and going on later to win a silver in the 50 m freestyle in Leeds.

After his initial victories, he rose through the ranks and broke the decade-long 50 m butterfly standing record twice, in the semis and final, giving him his first British Gas Swimming Championship title at the age of 19.

He went on the break his personal best times for the 50 m and 100 m freestyle events, earning himself a silver and bronze, as well as a spot on the British team for the 2013 World Championships being held in Barcelona.

[13] The following month, Proud won a gold in the 4 × 100 m medley relay and a bronze in the 50 m butterfly for Team GB at his first European Championships.

At the 2017 World Aquatics Championships, Proud qualified as the fourth fastest swimmer at the 50m butterfly semi-finals, but won a surprise gold at the final with a national record time of 22.75 seconds.

[17] At the 2018 Commonwealth Games held at the Gold Coast, Australia, Proud won silver as part of the relay team with David Cumberlidge, Jarvis Parkinson and James Guy that came second in the 4 × 100 m freestyle events.

After winning the 50 metres freestyle at the 2024 Aquatics GB Swimming Championships, Proud sealed his place at the 2024 Summer Olympics.