Robertson is the son of Shannon Robertson and grew up in Toronto. He played baseball and soccer growing up but decided to focus on basketball in eighth grade. He attended Vaughan Secondary School, where he was a bench player on a team led by Andrew Wiggins. Robertson followed his AAU coach Shane James to Thornlea Secondary School to try to get more playing time. As a senior, he posted 22 points, five assists and two steals per game. Even so, he was lightly recruited, and only received his lone Division I offer from Canisius after assistant coach Mike Mennenga came to scout a different player. Other college coaches liked his shooting but thought he was skinny and frail.
College career
Robertson redshirted his freshman season at Canisius, hitting the weight room and staying in the gym late. Campus police sometimes told Robertson to leave the gym. In his redshirt freshman year, Robertson scored 6.8 points per game and was named to the MAAC All-Rookie Team. Throughout his Canisius career, he developed into a productive 3-point shooter and potent scorer. He averaged 14.1 points per game as a sophomore in 2015-16. He scored 16 points in a 93-69 loss to Kentucky on November 13, 2016. As a redshirt junior, Robertson averaged 16.1 points, 3.1 rebounds and 2.4 assists per game. He was named to the Second Team All-MAAC. After his junior season, he opted to transfer to Missouri, choosing the Tigers over Georgia Tech because he liked Cuonzo Martin’s coaching philosophy. Robertson expected to be a three-point specialist playing alongside Michael Porter Jr., however his role expanded when Porter was injured in the first game. Robertson scored 23 points in a win against Wagner on November 13, 2017, shooting 5 for 8 on three-pointers. In a victory over St. John's on November 24, he posted 17 points and five assists. He organized late night team meetings after the team dropped three straight SEC games. Missouri rebounded to record a five game winning streak, during which time Robertson scored 20.6 points per game. Over this stretch he won two SEC Player of the Week awards. As a graduate transfer at Missouri, Robertson led the Tigers in scoring with 16.6 points per game while shooting 43 percent from behind the arc. At the conclusion of the regular season he was named to the First Team All-SEC. Robertson competed in a 3-on-3 tournament after the season alongside other SEC players.
