Thomson comes from a family with a long history of field hockey. Both his parents, Doug and Candy, represented Canada at the Olympic Games. Thomson is following in their footsteps and began playing at the age of 8 with the Hawks Field Hockey Club.
Harris made his first appearance with the Jr. Development Squad in San Diego against the USA in January 2016. He has since represent Canada at the 2016 Jr. Pan American Championships in Toronto at which he won a silver medal.
Thomson currently resides in Vancouver where he is a student at the University of British Columbia. He continues to pay his club hockey with the Vancouver Hawks FHC.