She represented the United States at the 2020 Summer Olympics and won a silver medal in the team event.

[10] In August McCallum competed at the National Championships where she placed fourth in the all-around, behind Simone Biles, Morgan Hurd, and Riley McCusker.

[11] On August 20 McCallum was named to the team to compete at the Pan American Championships alongside Jade Carey, Trinity Thomas, Kara Eaker, and Shilese Jones.

[16] The following day she was named to the team to compete at the 2018 World Championships alongside Biles, Hurd, McCusker, Eaker, and alternate Ragan Smith.

[20][21] In January, it was announced that McCallum would represent the USA at the American Cup alongside first year senior Leanne Wong in March.

[26] At the 2019 U.S. National Championships, McCallum competed all four events on the first day of competition but counted two falls and ended the night in ninth place, tied with MyKayla Skinner.

[28] In September McCallum competed at the US World Championships trials where she placed sixth in the all-around behind Simone Biles, Sunisa Lee, Kara Eaker, MyKayla Skinner, and Jade Carey after falling off the uneven bars.

The following day she was named to the team to compete at the 2019 World Championships in Stuttgart alongside Biles, Lee, Eaker, Skinner, and Carey.

Additionally, she placed ninth on floor exercise and tenth on uneven bars, but was not named a reserve athlete for either final due to both Biles and Lee qualifying above her on those two events.

[31] In early February, it was announced that McCallum was selected to represent the United States at the Birmingham World Cup taking place in late March.

[33] In November, McCallum signed her National Letter of Intent with the University of Utah, intending to start in the 2021–22 school year.

[35] In May McCallum competed the all-around at the U.S. Classic where she placed fourth behind Simone Biles, Jordan Chiles, and Kayla DiCello.

[38] 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.

After the first rotation Biles withdrew from the competition and the United States finished second behind the Russian Olympic Committee athletes.