Thom was born on June 13, 1986, in Burnaby, British Columbia, with spina bifida and hydrocephalus causing fluid build-up in the brain. As a result, he had four operations in the first 10 days of his life and although he had learned to walk by age two, a growth spurt meant he required further surgery for a tethered spinal cord. Thom and his family moved to Mississauga when he was four and he began wheelchair racing at the age of 10 after being inspired by the 1500m during the 1996 Summer Paralympics. His father, Ken, coached him until his death in 2017 as a result of a scuba diving accident.