Shields was born in Panama City, Florida, while his father was stationed at Tyndall Air Force Base.

He helped his university team, the California Golden Bears, finish second to the Texas Longhorns in the championships.

[7][8] He was the first champion from the California Golden Bears in the 100-yard backstroke at an NCAA Division I Swimming and Diving Championships.

At the 2011 Summer Universiade in Shenzhen, Shields earned the first international medal of his career, a gold, in the 4×200-meter freestyle relay for his contribution in the heats.

[10] Shields earned his first individual medal, a silver, in the 100-meter butterfly, finishing behind compatriot Tim Phillips.

[12] Shields finished first in the 100-yard butterfly with a time of 44.76 and first in the 100-yard backstroke in 44.86 seconds at the 2012 NCAA Division I Men's Swimming and Diving Championships.

[9] At the 2012 U.S. Olympic Team Trials, Shields finished fourth place in the 100-meter butterfly behind Michael Phelps, Tyler McGill, and Ryan Lochte, with a time of 51.86.

[19][20] His team, California Golden Bears, finished second in the year's championships to the University of Michigan.

[23][24] Bringing 2013 to a close, Shields broke his American record of 1:51.31 in the 200-meter butterfly with a time of 1:50.61 at the 2013 Duel in the Pool in Glasgow, Scotland.

[32] At the 2016 Summer Olympics in Rio de Janeiro, Brazil, Shields won a gold medal for his efforts as part of the 4×100-meter medley relay.

[35] In December 2016 at the US Winter National Championships in Atlanta, Georgia, Shields became the first American to swim the 100-yard butterfly faster than 44 seconds.

[54] In the evening, Shields helped advance the 4×100-meter medley relay to the final with Hunter Armstrong, Andrew Wilson, and Blake Pieroni.

[59][60] Shields decided to partake in the short course meters 2021 FINA Swimming World Cup starting at the first stop in Berlin, Germany where he was called a standout swimmer to watch in the 100-meter butterfly race by SwimSwam.

[66][67] In addition to the events he medaled in, Shields also competed in the 100-meter backstroke where he placed 12th in the prelims heats with a time of 53.78 seconds.

[86] His performances overall in Budapest earned him the position of top scoring male athlete at the World Cup stop.

[89] Shields kickstarted competition with the first race of the prelims heats on day one, the 100-meter butterfly, swimming a 52.44, ranking fourth overall and qualifying for the final.

[89][94] In the timed final of the 200-meter butterfly on day two, Shields placed third with a time of 1:43.11, winning the bronze medal in the event a few seconds after gold medalist Daiya Seto of Japan and five hundredths of a second after silver medalist Kregor Zirk of Estonia touched the wall.

[97] In his second event of the morning, Shields ranked eighth of the prelims heats of the 100-meter backstroke with a 57.87, qualifying for the final.

[99][100] A short while later, Shields wrapped up his competition in Doha by winning the silver medal in the 100-meter backstroke with a time of 51.12 seconds in the final.

[100][101] FINA noted Shields as someone to watch in his competitions at the fourth and final stop of the World Cup circuit, held at the Palace of Water Sports in Kazan, Russia, with his best times in the 50-meter butterfly and 100-meter butterfly from stops one to three of the World Cup garnering him the term "US rocket".

[102] On day one of competition, October 28, Shields ranked third in the prelims heats of the 100-meter butterfly and advanced to the final in the evening with his time of 51.31 seconds.

[103] He won the gold medal in the final, swimming a 49.20 to finish over eight tenths of a second ahead of the next fastest competitor.

[104][105] Speaking to FINA about his first win of the stop, his competitors, and his motivation, Shields said: A good start of the competition.

We shall never cease working hard and take these medals as a motivation for the coming challenges, and there will be a lot of them in during the FINA Worlds in Abu Dhabi.

[108] He swam a time of 1:52.42 in the final to win the gold medal in the event nine tenths of a second ahead of Louis Croenen of Belgium.

[120] Shields was entered to swim the 4x100-meter medley relay later the same day with Ryan Murphy, Christopher Rothbauer, and Maxime Rooney.

[124] By the end of the 2021 season, Shields had earned 666.8 most valuable player points in eighteen matches as part of the International Swimming League since its beginning in 2019, ranking sixth out of 488 competitors.

[125] For his individual events at the 2021 World Short Course Championships in Abu Dhabi, United Arab Emirates in December, USA Swimming named Shields to the US roster in the 50-meter and 100-meter butterfly.

[134] He followed up his relay performance with a time of 22.88 in the prelims heats of the 50-meter butterfly, which qualified him for the semifinals ranking fourteenth.