Green was born Nov. 16, 2000 in Sydney, New South Wales. He was born to Australian mother Cahla and American-born father Delmas. Green began playing basketball at five years old, in suburban Syndey, when his mother began coaching him. Green also played Australian rules football, track athletics, rugby, soccer and swimming. By the time he was 10 years old, he had been selected to represent his home state in nine separate sports. Green loved playing Australian rules football. In November 2014, he and his family moved to Phoenix, Arizona. After impressing at his first American High School, Mountain Ridge, he transferred to IMG Academy in Florida, where he became a five-star draft prospect. During the final game of Green's high school career, he led IMG to a national title and was named MVP of the championship game. For college, he went to Arizona. After averaging 12.0 points, 4.6 rebounds, 2.6 assists and 1.5 steals as a freshman, Green declared for the 2020 NBA Draft.