Johnstone grew up in Woodbine, Maryland and with a family history of both of his parents playing in college, Johnstone was destined to play rugby. He began playing at 7 years old for the West Carroll Marauders in Maryland. Johnstone continued into high school playing for the Marauders High School team and later joined DMV United 7s side and was selected for Capital Selects Rugby. Before graduating high school, Johnstone was invited to play on the USA High School All-Americans team in 2017.
After graduating high school, he continued his rugby career overseas in New Zealand playing for Ngongotaha RFC.
Once he returned to the US, Johnstone began college at Mount Saint Mary’s University in Emmitsburg, MD. During his time at Mount Saint Mary’s, he played for Beltway Elite 7s and was selected to play for the USA U20s team in which he scored 3 tries in 3 matches.
Next, Johnstone joined the Colorado Raptors where he represented the side through 2020. Continuing in Colorado, Johnstone joined the World Rugby North America Rugby Academy before joining the then Colorado XO’s now American Raptors squad. During the 2021 7s season, he competed with the Denver Barbarians and Denver Select sides.