The Honourable John Keiller MacKay, PC, DSO, VD, QC (July 11 1888 - June 12 1970), served as the 19th Lieutenant Governor of Ontario from 1957 to 1963.

John Keiller MacKay was born in 1888 in the village of Plainfield in Pictou County, Nova Scotia, and was educated at the Royal Military College (1909), Saint Francis Xavier University (BA 1912) and Dalhousie University (LL.B 1922).

During World War I he served in, and later commanded, 6th Brigade, Canadian Field Artillery. He achieved the rank of Lieutenant colonel and was mentioned in dispatches three times. MacKay won the Distinguished Service Order in 1916 at the Battle of the Somme and in 1918 was seriously wounded at Arras. He left the military after the war but was involved in the formation of the Royal Canadian Legion in 1925 and was its first National Vice-Chairman.

He was called to the Nova Scotia bar in 1922 and the Ontario bar in 1923. He was a member of a law firm in Toronto and was appointed a King's Counsel in 1933. He was appointed to the Supreme Court of Ontario in 1935 and 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.

He was married to Katherien Jean MacLeod and had three sons. He was a freemason.