Michelsen won the singles and doubles title at the 2022 Easter Bowl, becoming the first American champion in both disciplines since Donald Young in 2006.

[4] In August 2022, Michelsen made his Grand Slam doubles debut at the US Open playing alongside Sebastian Gorzny, losing in the first round.

In June, he made his ATP debut at the 2023 Mallorca Championships as a lucky loser, losing in the first round to eventual champion Christopher Eubanks.

Later that month, he reached his first final on the ATP Tour at the 2023 Hall of Fame Open in Newport,[5] where he lost to second seed Adrian Mannarino.

[7] Michelsen turned professional two weeks later, foregoing his college eligibility[4] and in August, he made his debut at a Major when he received a wildcard into the men's singles event at the US Open.

[13] He reached the third round at the 2024 Australian Open defeating wildcard James McCabe and 32nd seed Jiří Lehečka on his debut at this Major.

[6] Michelsen reached a consecutive final at the 2024 Hall of Fame Open, defeating Reilly Opelka in the semifinals[29][30] before losing to Marcos Giron.

[32][33] At the next North American swing tournament, the 2024 Winston-Salem Open, he reached the semifinals defeating Márton Fucsovics, Zizou Bergs, and Christopher Eubanks.

[39] At the 2025 Australian Open, Alex Michelsen faced again 11-seed Stefanos Tsitsipas and won in four sets to secure his first top 15 win in a Grand Slam.

[40][41] Next he defeated local wildcard James McCabe and 19th seed Karen Khachanov to reach the fourth round of a Grand Slam for the first time in his career.