At the 2023 NCAA Division I Men's Swimming and Diving Championships, he won a bronze medal in the 4×200-yard freestyle relay.

[1][2][4] Collegiately, he started competing for the University of Michigan Wolverines in autumn 2018 while pursuing studies for an undergraduate degree in business administration, graduating in 2022.

[6] He represented the United States at the 2017 World Junior Championships in Indianapolis, winning the silver medal in the 4×200-meter freestyle relay.

[22] Five days after the end of the Championships, Callan swam a 3:56.30 and placed fifth in the 400 meter freestyle at the 2022 Pro Swim Series in Westmont, Illinois.

[24] He placed fourth in the final with a time of 1:49.83, finishing 1.20 seconds behind fellow 2020 US Olympic team member Jake Mitchell.

[25] Leading up to the 2022 NCAA Division I Championships following the Pro Swim Series in Westmont, Callan announced he was staying at the University of Michigan to compete the following collegiate season as a fifth year, also called a super senior.

[27] On the second day of the 2022 US International Team Trials in Greensboro, North Carolina in April, Callan ranked 14th in the prelims heats of the 200 meter freestyle with a time of 1:48.76 and qualified for the evening b-final.

[35] The following World Cup stop, held in his home county at the Indiana University Natatorium in Indianapolis, Callan placed nineteenth in the 400 meter freestyle on day one with a time of 3:49.14.

[39] In his first individual event of the Championships the following day, the 500 yard freestyle on March 2, he placed fourth with a 4:12.45, finishing within a quarter of a second of the bronze medalist.

[42] On the final day, he placed eighteenth in the preliminaries of the 200 yard butterfly with a time of 1:46.81 before withdrawing from competition in the c-final.