2021

At age 16 in January, Knighton signed a sponsorship deal with Adidas in his junior year of high school, forgoing his remaining two years of amateur competition at Hillsborough High.^[2] On May 2, he broke the 10-second barrier over 100 meters at the PURE Athletics Sprint Elite Meet in Clermont, Florida, with a time of 9.99 seconds, but the wind was over the +2.0 meters per second velocity limit (+2.7) for record consideration.^[8]

On May 31, the 17-year-old set the world under-18 best in the boys' 200 meters in a time of 20.11 seconds, breaking Usain Bolt's best by two hundredths of a second.^[9][10] At the US Olympic Trials he would improve that time to 20.04 s in the first round on June 25, and then again to 19.88 s in the semi-finals the following day, breaking Bolt's world U20 record by five hundredths of a second. He then improved his own record to 19.84 seconds in the final on June 27, qualifying for the postponed 2020 Tokyo Summer Olympics.

At the Tokyo Games, Knighton became the youngest male to represent the United States in track and field since Jim Ryun in 1964.^[11] On August 3, he finished first in his 200-meter Olympic semi-final heat and qualified for an automatic spot in the final to be run the next day.^[12] In the final he finished in fourth with a time of 19.93 seconds.^[11]

2022

On April 30, Knighton set an unratified world junior record in the 200 m at the LSU Invitational in Baton Rouge running a time of 19.49 seconds.^[13] He achieved 19.69 s at the USA Outdoor T&F Championships in June. Knighton later on went on to place third in the event at the 2022 World Athletics Championships, in Eugene, USA, becoming the youngest ever individual sprint medalist in Championship history.^[14][15] He also became the youngest winner of a Diamond League race with his 200 m victory on September 2 in Brussels.^[16]