Although he was born on Canada's East Coast, Harry Watson spent the first few years of his life in England. When his family moved back to Canada, Watson learned to skate and play hockey on the frozen rivers and ponds, as did many of his neighbourhood friends. When the family moved to Toronto in 1913, Harry signed on to play for the Whitby Athletics of the OHA. He moved on to St. Andrews College in the OHA Jr. league and was named a First Team All-Star in 1915. By the 1916-17 season, he was leading the OHA Sr. league with 18 goals while playing eight games with Toronto Aura Lee.
Watson joined the Flying Corps and served overseas during the war before returning to hockey action with the Toronto Dentals of the OHA Sr. league. He made it back in time for one playoff game against the Hamilton Tigers in which he scored a goal in a losing cause.
In 1919, the Toronto Granites hockey team was formed by the eponymous curling club and Watson was their star player, leading them to Allan Cup titles in 1921-22 and 1922-23. The Granites went on to represent Canada at the 1924 Olympics and win the gold medal, with Watson scoring an astounding 37 goals in five games against the competition. Individual awards followed Watson as well; he was named to the OHA Sr. Second All-Star Team in 1920 and made the First Team All-Stars in 1922 and 1923. He was also named as the OHA Sr. most valuable player in 1922 and 1923.
The fame of the Granites and their exploits did not go unnoticed by the National Hockey League. A number of the Granites players, notably Watson, Hooley Smith, and Dunc Munro, were made offers to turn professional. Smith and Munro agreed to turn pro, but Harry Watson clung proudly to his amateur status.
Harry Watson was inducted into the Hockey Hall of Fame in 1962.