[4][5] At the 2019 FINA World Junior Swimming Championships, Alexy earned a gold medal with Team USA in the 4 × 100 m freestyle relay.

On Day 4, Alexy swam a new personal best of 40.88 in the preliminaries of the 100 yard freestyle, taking the top seed entering the A final.

In April, at the TYR Pro Swim Series - Westmont meet, Alexy won the 50 meter freestyle in a personal best time of 22.09.

He outswam Michael Andrew (swimmer) by a hundredth of a second, denying him a spot on the World Championship roster.

His breakthrough season culminated at the 2023 World Aquatics Championships in which he earned five medals for Team USA in his senior international debut.

Additionally, he contributed to Cal's victory in the 800 Free Relay with a flying start split of 1:30.50, setting a new NCAA record of 6:02.26.

On Day 3, Alexy made his NCAA Championship debut in the 200 Freestyle and recorded a time of 1:29.75 in the final to place second.

Cal placed second behind the Arizona State Sun Devils; out of the 444.5 points they scored, Alexy contributed 49 of them with his individual performances and 136 through the relays.

During the first finals session of the Olympic swimming competition, the team of Alexy, Guiliano, Armstrong, and Dressel earned the gold medal in the 4 × 100 meter freestyle relay.

In the final, Alexy placed 7th with a time of 47.96, behind a world record performance by Pan Zhanle of 46.40, and ahead of his teammate Giuliano.

Alexy also earned a silver medal by swimming the freestyle leg in the preliminaries of the 4 × 100 meter medley relay.