John Keiller MacKay was born in 1888 in Pictou County, Nova Scotia, and was educated at St Francis Xavier University, Royal Military College and Dalhousie University. During World War I, MacKay won the Distinguished Service Order. He was called to the Nova Scotia bar in 1922, and became a member a law firm in Toronto until his appointment to the Supreme Court of Ontario in 1935. He was appointed to the Court of Appeal in 1950. MacKay served as Lieutenant Governor of Ontario from 1957 to 1963, and opened the Lieutenant Governor's New Years Levee to the general public for the first time. He died in Toronto in 1970.