2014–15: Emergence and World titles

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.

He competed for England in the 4x100-metre medley relay and the 400-metre freestyle events at the 2014 Commonwealth Games where he won a gold and bronze medal respectively.^[8]

At the 200-metre freestyle event at the 2015 World Aquatics Championships, Guy won the gold medal beating Olympic silver medallist Sun Yang of China by six hundredths of a second.^[9][10]

At the same event, Guy struck gold again in the 4 × 200 m freestyle relay. Team GB was third at the last take over, when Guy took over. He swam his 200 in a 1:44.7, the quickest time across the pool and securing Great Britain the gold medal for the first time at the worlds in this event.^{[citation needed]}

2016: Relay success in Olympics debut

At the Rio Olympics in 2016, Guy won silver in the 4x200m freestyle relay with Stephen Milne, Duncan Scott, and Dan Wallace, and a further silver in the 4x100m medley relay with Chris Walker-Hebborn, Adam Peaty and Duncan Scott. He came fourth in the individual 200m freestyle event.^[1]

2017: World relay gold and transition to butterfly

In February 2017 Guy, alongside coach Jol Fincke, moved his training base to the National Centre for Swimming at the University of Bath.^[11]

At the 2017 World Aquatics Championships. Guy won gold in the 4x200m freestyle with Duncan Scott, Stephen Milne and Nick Grainger in a time of seven minutes 1.70 seconds.^[12] He shared the 100m butterfly bronze with Joseph Schooling of Singapore, both recording a time of 50.83 seconds.^[13] In the 4x100m medley relay, he again won silver with the same Olympic line-up of Walker-Hebborn, Peaty and Scott.^[14]

2018: Europeans, Commonwealth relay success and final 400m free

At the 2018 Commonwealth Games held at the Gold Coast, Australia, Guy was part of the teams that won silver in the 4x200m freestyle, the 4x100m freestyle,^[15][16] as well as in the 4x100m medley, finishing behind Australia in all three.^[17] He also won a silver in the 100m butterfly,^[18] and a bronze on 400m freestyle, after which he announced that he would no longer swim the 400 metre freestyle event in competition, focusing instead on the 100 metre butterfly.^[19]

At the 2018 European Championships, Guy swam the anchor leg in the 4 × 200 metre freestyle relay, and won the gold medal with Duncan Scott, Thomas Dean and Calum Jarvis.^[20] The following day he won a second gold as part of the team in the mixed 4 × 100m medley,^[21] then added a third on the final day of the competition by winning as part of the British team the men's 4 × 100 metre medley relay. Individually, he took the bronze medal in the 100 metres butterfly on the final night.^[22]

2019: Further World Championships success

At the 2019 World Aquatics Championships held in Gwangju, South Korea, Guy won a bronze as part of the GB team in the 4 × 100 m mixed medley relay.^[23]

In the men's 4 × 100 metre medley relay, he was part of the team together with Adam Peaty, Duncan Scott and Luke Greenbank. They won the final in a European record time of three minutes, 28.10 seconds, and the win is Britain's first gold medal in the event at the World Championships.^[24][25]

2021: European Championships and Olympic gold

At the European Championships held in Budapest in May 2021, Guy won three gold medals as part of the British team in the men's 4 x 100 m medley, mixed 4 × 100 metre medley, and mixed 4 × 200 metre freestyle relays.^[26][27][28] He also won two silvers in both the men's 4 × 100 metre and 4 × 200 metre freestyle relays,^[29][30] as well as an individual bronze in 100m butterfly.^[31]

At the Tokyo Olympics, Guy swam the second leg in the final of the men's 4 × 200 metre freestyle relay, and won his first Olympic gold with Tom Dean, Duncan Scott, and Matt Richards in a time of six minutes 58.58 seconds.^[32] He then won a second gold, swimming the butterfly leg in the mixed 4 × 100 metre medley relay, and set a world record time of 3 minutes 37.58 seconds together with Kathleen Dawson, Adam Peaty, and Anna Hopkin.^[33]

Guy was appointed Member of the Order of the British Empire (MBE) in the 2022 New Year Honours for services to swimming.^[34][35]

2022

At the 2022 World Aquatics Championships held in Budapest, Guy won two bronzes as part of the team, in Men's 4 × 100 metre medley relay and Men's 4 × 200 metre freestyle relay.^[36] In 2022, he was part of the team that won the gold medal in the Men's 4 × 100 metre medley relay at the 2022 Commonwealth Games in Birmingham.^[37]