[1] He played basketball for Bill Crothers Secondary School in Markham, Ontario, for grade 9 from 2017 to 2018 [2] and transferred to Thornlea Secondary School in Thornhill, Ontario, for grade 10, where he helped the team win the National Junior Circuit title in March 2019.

[3] Miller transferred to Wasatch Academy in Mount Pleasant, Utah, to face stronger competition but received limiting playing time.

He transferred to Victory Rock Prep in Bradenton, Florida, before missing several months with a broken right wrist in November 2020 that required surgery.

Miller led Fort Erie to an Ontario Scholastic Basketball Association title and was named league most valuable player.

[15][16] Miller won a silver medal with Canada at the 2019 FIBA Under-16 Americas Championship in Brazil, averaging 4.2 points per game.