The 2021 AMA Supersport Championship season was the demonstration of power for Kelly in the category.
He achieved twelve victories (one in Virginia, Pittsburgh, New Jersey[5] and Alabama and two in Atlanta,[6] Washington,[7] Laguna Seca[8][9] and Minnesota),[10] five second places and one sixth place was his worst finishing position of the season over 18 races.
With his second-place finish in New Jersey Race 2, Kelly became the AMA Supersport champion one round before the end of the championship.
[11] In 2019, he made his Grand Prix motorcycle racing debut in the Moto2 category; he entered the Valencian Community Grand Prix replacing Spanish racer Iker Lecuona in American Racing KTM.
On September 18, 2021, it was announced that Kelly was officially signed by American Racing in Moto2 on a two-year contract.