Guy came to international prominence when he won two World Championship gold medals in the 200-metre freestyle and 4×200-metre freestyle relay event at the 2015 World Aquatics Championships. A prodigious relay swimmer, in 2016, he won silver in the 4x200m freestyle relay and the 4x100m medley relay at the Rio Olympics, finishing 4th in the individual 200 metre freestyle event. He helped defend the men's 4 x 200 metre freestyle relay world title in 2017, and swam the butterfly leg to help Great Britain to gold in the 4 x 100 metre medley relay at the 2019 World Aquatics Championships. Guy is a Commonwealth Games and seven-time European champion across various relay events.