McCallum is the 2018 Pan American and 2018 Pacific Rim individual all-around champion, the 2018 Pan American uneven bars champion, and was a member of the U.S. gymnastics team that won gold at the 2018 and 2019 World Championships and the 2018 Pan American Championships.
At the Olympic Games McCallum performed the all-around during qualifications and helped the USA qualify to the team final in second place behind the athletes from Russia. She finished qualifications in thirteenth place; however she did not advance to the final due to two-per-country limitations as Biles and Lee had placed higher. During the team final McCallum competed on all four apparatuses. After the first rotation Biles withdrew from the competition and the United States finished second behind the Russian Olympic Committee athletes.
McCallum made her NCAA debut on January 7 at the Best of Utah meet where she competed the all-around to help Utah win. She put up the highest vault score of the night, a 9.90, alongside teammate Jaedyn Rucker. The following week McCallum once again competed the all-around to help Utah win against Oklahoma. She recorded the highest all-around and floor exercise scores of the night with a 39.675 and 9.975 respectively. As a result she was named Pac-12 freshman of the week. On February 4, in a meet against UCLA, McCallum earned her first collegiate perfect ten on the uneven bars.
At the Pac-12 Championships McCallum helped Utah win their second consecutive team title. Individually she placed second in the all-around behind Olympic teammate Jade Carey. She earned her second perfect ten on the uneven bars to outright win the title and co-won the title on floor exercise alongside Carey.
