Fultz was born on May 29, 1998 in Prince George's County, Maryland, the younger of two children.
His father abandoned the family, so he relied heavily on his mother, Ebony.
Impassioned by basketball, Fultz was mentored in his early years by a local instructor named Keith Williams.
In 2012, he began attending DeMatha Catholic High, an all-boys prep school as well as basketball powerhouse.
He had shown promise on the DeMatha Stags' freshman team; but as a sophomore, the 5-foot-9-inch (1.75 m) guard was cut from the varsity roster.
He was overlooked due to his awkward gait, having the appearance of "Bambi"; as assistant coach Cory McCrae explained, "He was long and lanky, and he’d walk and his knees would be hurting".
However, Fultz outplayed the competition at the junior varsity level, making it apparent that he did not belong there.

Assistant coach Raphael Chillious of the Washington Huskies was the first to notice Fultz's potential, describing him as an athlete with the makings of an NBA All-Star-caliber player if he continued to grow.
At the start of his junior season, Fultz stood 6 feet 3 inches (1.91 m).
As a starter for DeMatha's varsity team, he averaged 16.8 points, 7.9 rebounds, and 4.3 assists per game and was named the Player of the Year in the highly-competitive Washington Catholic Athletic Conference.
Among the highlights of his junior campaign were the game-winning foul shots he made in a 16-point performance while being guarded by Jayson Tatum of Chaminade and the triple-double he posted in a matchup against Roselle Catholic at the Hoophall Classic.
His recruiting stature abruptly soared during this season, with Rivals.com ranking him number 24 in the Class of 2016 and over 20 college programs offering him scholarships.

On August 21, 2015, Fultz committed to the University of Washington.
He concluded his high school career as he recorded the Stags' single-season record for assists with 278 and led the team to their second consecutive conference championship.
At the McDonald's All-American Game on March 31, 2016, Fultz tallied 10 points, 6 rebounds, and 4 assists.
At the Jordan Brand Classic come April, Fultz accumulated 19 points in a 131–117 East team victory.
During the summer, he was selected to Team USA's FIBA Americas Under-18 roster.
With Fultz as its leading scorer, the team progressed to the gold medal championship game; and in dominant fashion, Fultz posted 23 points and earned the tournament's MVP award for his performance.

By the end of his senior year, Fultz was rated as a five-star recruit and ranked the seventh-best overall recruit and third-best guard in the 2016 high school class.
At DeMatha, Fultz was a basketball teammate and friend of Chase Young, who was later selected by the Washington Football Team, then known as the Redskins, second overall in the 2020 NFL Draft.
Fultz was born on May 29, 1998 in Prince George's County, Maryland, the younger of two children. His father abandoned the family, so he relied heavily on his mother, Ebony. Impassioned by basketball, Fultz was mentored in his early years by a local instructor named Keith Williams. In 2012, he began attending DeMatha Catholic High, an all-boys prep school as well as basketball powerhouse. He had shown promise on the DeMatha Stags' freshman team; but as a sophomore, the 5-foot-9-inch (1.75 m) guard was cut from the varsity roster. He was overlooked due to his awkward gait, having the appearance of "Bambi"; as assistant coach Cory McCrae explained, "He was long and lanky, and he’d walk and his knees would be hurting". However, Fultz outplayed the competition at the junior varsity level, making it apparent that he did not belong there.
Assistant coach Raphael Chillious of the Washington Huskies was the first to notice Fultz's potential, describing him as an athlete with the makings of an NBA All-Star-caliber player if he continued to grow. At the start of his junior season, Fultz stood 6 feet 3 inches (1.91 m). As a starter for DeMatha's varsity team, he averaged 16.8 points, 7.9 rebounds, and 4.3 assists per game and was named the Player of the Year in the highly-competitive Washington Catholic Athletic Conference. Among the highlights of his junior campaign were the game-winning foul shots he made in a 16-point performance while being guarded by Jayson Tatum of Chaminade and the triple-double he posted in a matchup against Roselle Catholic at the Hoophall Classic. His recruiting stature abruptly soared during this season, with Rivals.com ranking him number 24 in the Class of 2016 and over 20 college programs offering him scholarships.
On August 21, 2015, Fultz committed to the University of Washington. He concluded his high school career as he recorded the Stags' single-season record for assists with 278 and led the team to their second consecutive conference championship. At the McDonald's All-American Game on March 31, 2016, Fultz tallied 10 points, 6 rebounds, and 4 assists. At the Jordan Brand Classic come April, Fultz accumulated 19 points in a 131–117 East team victory. During the summer, he was selected to Team USA's FIBA Americas Under-18 roster. With Fultz as its leading scorer, the team progressed to the gold medal championship game; and in dominant fashion, Fultz posted 23 points and earned the tournament's MVP award for his performance. 
By the end of his senior year, Fultz was rated as a five-star recruit and ranked the seventh-best overall recruit and third-best guard in the 2016 high school class. At DeMatha, Fultz was a basketball teammate and friend of Chase Young, who was later selected by the Washington Football Team, then known as the Redskins, second overall in the 2020 NFL Draft.
Markelle Fultz