In 2015, Jeffery became the third youngest player to achieve the Grandmaster title at age 14. He came in second at the U.S. Junior Closed Chess Championship in July 2015 which qualified him for his first appearance in the U.S. Chess Championships. By 2016 he entered the top 10 players in the world under age 20. Recently, Jeffery has shattered the 2700 rating barrier, with a current rating of 2712. He continues to have impressive results such his run into the quarterfinals in the 2019 FIDE World Cup, where he upset impressive grandmasters like Anish Giri and Jan-Krzystzof Duda.