She is the youngest woman in history to hold Olympic and world titles in an individual track and field event.

[4] On February 24, 2019, Mu broke the American women's record at the 600 meter event at the 2019 USA Indoor Track and Field Championships with a time of 1:23.57.

[7] However, Mu's time was slower than the 50.36 s set by fellow American Sydney McLaughlin, which was not able to meet the standards for world record ratification.

On April 17 in Waco, Texas, running outdoors, Mu set the 800 meter USA collegiate record with a time of 1:57.73.

In her individual event, Mu led from gun to tape in a dominant showing, finishing clear ahead of Keely Hodgkinson and compatriot Raevyn Rogers.

[15] In her first post-Olympic race at the Prefontaine Classic, she set even better American 800 m record of one minute 55.04 seconds despite running by herself over the final lap, also the all-comers' record, making her the second fastest U20 woman ever after Pamela Jelimo and putting her eighth on the world all-time list.

[16][17] In that record-breaking season Mu competed 36 times (including rounds) and triumphed in 35 races to be voted World Athletics Female Rising Star of the Year.

At September's 2023 Prefontaine Classic, she won the Diamond League final in a time of 1:54.97 to set an American record and defeat Moraa and Hodgkinson.