Johnson was born in Chesterfield, Virginia, to Chris and Rochelle Johnson in 1999. He has two brothers, Kaleb and Kyle, and one sister, Kristyn. Kaleb played college football at Rutgers and later competed in the American Football League. Keldon attended Park View High School in South Hill, Virginia. He later transferred to Oak Hill Academy in Mouth of Wilson, Virginia. There he starred on the basketball court, averaging 20.5 points, 6.3 rebounds, 3.2 assists and 2.2 steals as a senior. Johnson was subsequently named the Virginia Gatorade High School Player of the Year. In addition, he was selected to the USA Today All-USA Second Team. He was also named to both the McDonald's All-American Game and the Jordan Brand Classic Game. During the summer of 2017, Johnson played for the United States Men's National Under-19 basketball team. Learn more about Johnson by following him on Twitter and Instagram (@keldonjohnson).