She moved to the Chelsea Piers Aquatic Club in Connecticut in 2017 and swam there for the last two years of high school.

[4] A month later, at the 2016 U.S. Winter Junior Championships, Douglass tied Simone Manuel's 15–16 national age group record in the 50 yard freestyle with a time of 22.04.

[7] Due to her national age group records in the 50 yard freestyle and her top-ranked times in the breaststroke and individual medley events, she was named SwimSwam's #2 girls' recruit in their class of 2019 rankings.

[10] Although Douglass had a national age group record heading into college, she broke through as an elite swimmer in her freshman year at Virginia with coach Todd DeSorbo.

[12] At the 2020 ACC Championships, Douglass won the 200 yard individual medley title in a time of 1:51.36, making her the fastest freshman ever in the event and the fourth-fastest in history.

[17] Douglass started off 2021 ACC Championships by breaking the NCAA record in the 200 medley relay with her teammates Caroline Gemlich, Alexis Wenger, and Lexi Cuomo.

[18] Douglass won her first NCAA title when she beat Michigan's Maggie Mac Neil by four-hundredths of a second in the 50 yard freestyle with a time of 21.13.

However, in the 200 m individual medley, Douglass finished second to Alex Walsh and swam a personal best time of 2:09.32.

[20] At the 2020 Olympic Games, Douglass swam the top time in both the preliminary and semifinal rounds of the 200 m individual medley.

Douglass beat fourth-place finisher Abbie Wood by 0.11 seconds, being behind after the first 150 meters of the race and passing her in the freestyle leg to grab the bronze medal in a personal best time of 2:09.04.

At the 2021 Tennessee Invite, Douglass broke Sophie Hansson's ACC record in the 200 yard breaststroke, swimming a time of 2:03.58 to become the fourth-fastest performer ever.

She began the meet by breaking the NCAA, U.S. Open, and American record in the preliminary rounds of the 50 freestyle, swimming a 20.87.

On the final day of the meet, Douglass won the 200 breaststroke by over two seconds, clocking a 2:02.19 to break Lilly King's NCAA, U.S. Open, and American record.

"[25] The CSCAA and swimming news outlet SwimSwam both named Douglass the 2022 NCAA Female Swimmer of the Year.

[26] At the 2022 U.S. International Team Trials in Greensboro, North Carolina, Douglass qualified for the 200 m breaststroke at the World Championships in Budapest.

For the 4×100 m freestyle mixed relay, she was also awarded a bronze medal, as she competed in the heats and helped her country qualify for the final.

At the 2022 Short Course World Championships in Melbourne, Douglass won a gold medal in the 200 m individual medley, setting a new Americas record of 2:02.12 and swimming the second-fastest time ever over this distance after Katinka Hosszú in 2014 (2:01.86).

She also won the gold medal in the 200 m breaststroke by setting a new championship record, winning her second individual title in a global competition within three days.

At the 2022 Tennessee Invite, Douglass broke her own NCAA, U.S. Open, and American record in the 200 yard breaststroke, swimming a time of 2:01.87.

[28] At the 2023 Cavalier Invitational, Douglass further reset the U.S. Open and American record in the 200 yard breaststroke with a time of 2:01.43, becoming the fastest ever in the event by over a second.

[33] She won the 100 yard butterfly the next day in another American and U.S. Open record time of 48.46, edging out Olympic champion Maggie Mac Neil by five-hundredths of a second.

[34] On the final day, Douglass completed a clean sweep of her three individual events with a victory in the 200 yard breaststroke, clocking a time of 2:01.29 which was another American and U.S. Open record.

Her NCAA career was rated as one of the greatest of all time by various sporting and swimming news outlets, including SwimSwam.

[41][42] Afterward, she started studying for her master's degree in statistics at Virginia, researching data analytics in competitive swimming with Ken Ono.

[48] At the Golden Goggle Awards in November, Douglass was named the Female Athlete of the Year, along with Katie Ledecky.

She started off the meet by swimming in the heats and the final of the 4 × 100 m freestyle relay, helping the U.S. win the silver medal.

"Understated in temperament, underrated in toughness, impeccable in technique, Douglass was the hero America wanted and needed in the pool at these Paris Olympics.

On December 10, Douglass won gold in the 200 m individual medley, defending her 2022 title and breaking the world record.

Later that night, she swam in the final of the 4 × 100 m freestyle, helping the U.S. team win gold in a world record time.

Later that night, she swam the freestyle leg in the 4 × 100 m medley final, helping the U.S. win gold in a world record time.