He was a lawyer and judge, from Nova Scotia, as well as Premier of Nova Scotia, and became the 4th Prime Minister of Canada. He was a Conservative who made his mark, as the Minister of Justice, in 1885 with his sentencing of Louis Riel. Riel was sentenced to hang for leading the 1885 North-West Rebellion. Ill with kidney stones at the time of Riel's execution, Thompson made his first major speech to parliament during the subsequent debate, arguing that anyone who encouraged aboriginal Canadians to act against the state could not escape justice.
Sir John had been Prime Minister of Canada for only 2 years when he died suddenly of a heart attack at Windsor Castle. He was in England because Queen Victoria had just made him a member of her Privy Council. He was the second of two Canadian prime ministers to die in office, and the first of three who did not die in Canada.