In 2013, at the World Junior Championships in Dubai, Pieroni took the bronze in the 4 x 200m freestyle with the men's relay team.
2014 Junior Pan Pacific Championships
As a 18-year-old at the 2014 Junior Pan Pacific Swimming Championships in Maui, Hawaii, Pieroni won gold medals in the 4×100 meter freestyle relay (3:18.68) and 4×200 meter freestyle relay (7:21.36), as well as silver medals in the 100 meter freestyle (50.30) and 200 meter freestyle (1:48.85).[6]
At the 2016 World Championships in Windsor, Ontario, with the men's relay team, Pieroni won two silver and one bronze medals: silver in 4 x 50 m freestyle; silver in 4 x 200 m freestyle and bronze in 4 x 100 m freestyle.
Pieroni won two gold medals and a bronze medal at the 2017 World Championships in Budapest as part of the men's and mixed freestyle teams: gold in 4 x 100 m freestyle, gold in 4 x 100 m mixed freestyle and bronze in 4 x 200 m freestyle.
At the Tokyo Pan Pacific Championships in August 2018, Pieroni won gold with the men's relay team in the 4 x 200 m freestyle event. He also won the B-final of the 50-meter freestyle with a time of 22.22 seconds and ranked ninth across both finals heats.[3]
At the 2018 World Championships in Hangzhou, China, which took place in a short course (25m) pool in December, Pieroni returned home with three gold medals, having won the 200 m freestyle as well as 4 x 100 m freestyle and medley events, as part of the men's relay team.
2019 World Championships
At the 2019 World Championships in Gwangju, South Korea, Pieroni took home two gold medals as part of the 4 x 100 m and 4 x 100 mixed freestyle teams. He also won a bronze medal as part of the 4 x 200 m freestyle team.
Pieroni placed third in the 100m freestyle at the US Olympic Swimming Trials with a time of 48.16, qualifying him for the US Olympic Swimming team in the 4x100m freestyle relay.[8] Earlier in the meet, in the 200-meter freestyle, Pieroni ranked seventh overall with a time of 1:46.57 and qualified for his first event at the 2020 Summer Olympics, the 4x200-meter freestyle relay.[9]
At the 2020 Summer Olympics held in Tokyo, Japan, Pieroni competed in the prelims and final of the 4x100-meter freestyle relay.[10] In the final of the event, he won a gold medal swimming the second leg of the relay.[11][12] He and all of his prelims and finals relay teammates, Brooks Curry, Bowe Becker, Caeleb Dressel, and Zach Apple, won gold for their efforts.[10] It was the first medal Pieroni won swimming in the final of any event at the Olympic Games.[13] His performance was good enough in the eyes of the USA Swimming Foundation to include him in their nomination of the 4x100-meter freestyle relay for the 2021 Golden Goggle Award for "Relay Performance of the Year" solely because he swam on the finals relay, something he learned after he was excluded from the nomination of, and award itself to, the 2016 United States 4x100-meter freestyle relay for the 2016 Golden Goggle Award for "Relay Performance of the Year" when he only swam on the prelims relay and not in the final.[14][15]
In the prelims heats of the 4x100-meter medley relay on day seven, Pieroni swam the freestyle leg of the relay, helping the relay consisting of him, Hunter Armstrong, Andrew Wilson, and Tom Shields, finish seventh overall and advance to the final.[19][20] On the last day of competition, the finals relay placed first and all the members of the prelims relay and the finals relay won a gold medal for their efforts.[21][22]
2021 International Swimming League
The Toronto Titans selected Pieroni for their roster in the 2021 International Swimming League.[23] After injuring his knee during training leading up to the playoffs season and following the regular season of competition, Pieroni withdrew from competing in the playoffs for the year.[24][25]
As part of the pre-World Cup FINA highlights, Pieroni had a picture of him celebrating a win at the 2020 Summer Olympics published on the FINA website as part of their star athletes to appear in the World Cup collection of photographs.[26] Stop number one of the short course 2021 FINA Swimming World Cup, in Berlin, Germany, Pieroni started competition on day one in the prelims of the 50-meter freestyle with a swim of 21.75 that ranked him fifth overall and advanced him to the final.[27] Within the hour, Pieroni followed up his performance in the 50-meter freestyle with a 53.73 in the 100-meter individual medley and advanced to the final ranked second overall.[28][29] In the evening finals, Pieroni opted not to swim the 50-meter freestyle, focusing his efforts fully on the 100-meter individual medley where he swam a personal best time of 53.53 seconds and won the bronze medal in the event.[30] The following day, October 2, Pieroni advanced to the final of the 100-meter freestyle with his time of 47.25 in the morning prelims session.[31][32] In the final in the evening, he won the bronze medal, touching the wall third with a time of 46.74 behind Kyle Chalmers of Australia who won the gold medal and Jesse Puts of the Netherlands who won the silver medal.[33][34] The third and final day of competition in Berlin, Pieroni placed fourth in the final of the 200 meter freestyle with a time of 1:42.87 after qualifying for the final ranking fifth with his time of 1:44.29 from the prelims.[35][36]
Stop 2: Budapest
For the second World Cup stop, held in Budapest, Hungary, Pieroni was noted by FINA and Swimming World as one of the athletes from the United States to watch at the international competition.[37][38] On October 7, the first day of competition, he advanced to the final of the 100-meter individual medley ranked fifth overall and did not qualify for the final of the 50-meter freestyle, ranking tenth in the prelims heats.[39][40] In the 100-meter individual medley final later the same day, he just missed the podium, placing fourth with a time of 53.82 seconds.[41] Morning of day two, Pieroni ranked fourth overall in the 100-meter freestyle with a time of 47.44 seconds and qualified for the final later in the day.[42][43] In the final, he placed fifth with a time of 47.09 seconds in a talent-packed field, finishing after Kyle Chalmers, Vladimir Morozov of Russia, Kristóf Milák of Hungary, and Jesse Puts.[44] For his last race in Budapest, Pieroni swam a 1:49.59 and ranked 23rd in the 200-meter freestyle, not advancing to the final in the evening.[45][46]
Stop 3: Doha
Entering the days ahead of the third World Cup stop, in Doha, Qatar, Swimming World listed Pieroni as a United States roster standout from the list of athletes entered to compete at the stop.[47] He switched out the 50-meter freestyle and substituted in the 100-meter butterfly for his event entries line-up compared to the previous two World Cup stops, giving him the butterfly race, the 100-meter freestyle, 100-meter individual medley, and 200-meter freestyle races in Doha.[48]
Pieroni commenced competing in a duo of events on day one of competition with the prelims heats of the 100-meter butterfly in the morning where he ranked seventh with a time of 52.76 seconds and qualified for the final.[49] Finishing off his morning duo of events about 30 minutes later, Pieroni ranked eighth in the prelims of the 100-meter individual medley with a time of 54.61 seconds and advanced to the final.[50] In his first race of the evening, the final of the 100-meter butterfly, he placed sixth with a time of 52.34 seconds.[51] While Pieroni qualified for the final in the 100-meter individual medley, he decided to go with not racing in the final.[50][52] Rejuvenated for the prelims of the 100-meter freestyle the following morning, Pieroni swam a 48.15 and advanced to the final ranked fifth overall.[53][54] He lowered his time to a 47.59 in the final, placing fifth overall.[55][56] For his last event of competition in Doha, Pieroni did not start the race in the 200-meter freestyle, deciding not to swim the race.[57] Pieroni's scores from the first three stops were high enough for him to rank sixteenth out of all male competitors, and second out of male American competitors, for overall score across all four stops of the 2021 World Cup circuit.[58]
2022: Retirement
On August 26, 2022, swimming-centric news outlets Swimming World and SwimSwam announced Pieroni had retired from swimming and would not be returning to competition in the sport.[59][60]
As part of being a professional swimmer, Pieroni took the path less traveled and signed with Japanese sporting goods company Mizuno Corporation in 2018, making history by becoming the first American swimmer in history to do so.[65][66]
