Leonard Miller was born in
Scarborough,
Ontario, and played basketball,
volleyball and
golf during his childhood. He played basketball for
Bill Crothers Secondary School in
Markham, Ontario, for grade 9 from 2017 to 2018 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. 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 opted to play a
postgraduate season at Fort Erie International Academy in
Fort Erie, Ontario after growing 6' 4" guard to 6' 10" guard/wing over an 18-month period, and had no college offers at the time he made the decision. He became the team's star player and received over 25 offers from college programs. Miller led Fort Erie to an Ontario Scholastic Basketball Association title and was named league
most valuable player. On April 23, 2022, he declared for the
2022 NBA draft. On June 13, 2022, he withdrew from the draft and decided to play with the
NBA G League Ignite for the next season.
Recruiting Miller was considered a five-star
recruit by
Rivals and was ranked as the 2nd ranked prospect by
On3.com despite playing in Canada with On3 ranking him first with the highest upside in the recruiting class. On May 31, 2022, he announced that he would pursue professional options instead of playing
college basketball. ==Professional career==