He came to senior prominence in 2014, Guy laid down a signal of intent with a hat-trick of golds at the British Gas Swimming Championships, taking the touch in the 200m Free, 400m Free and 200m Fly, and breaking the British record in the 400m Free. He represented England at the 2014 Commonwealth Games, winning two relay medals, lowering the British record again to claim 400m Freestyle bronze for England. Thereafter, representing Great Britain, he took silver medals in the 400-metre freestyle at the FINA World Championships in both the short course (25 metre) and long course formats. On 4 August 2015, he won the 200-metre freestyle at the 2015 World Aquatics Championships, the first British swimmer to win the event in its history.