John Sandfield Macdonald, was the joint premier of the Province of Canada from 1862 to 1864. He was also the first premier of Ontario from 1867 to 1871, one of the four founding provinces created at Confederation in 1867. He served as both premier and attorney general of Ontario from July 16, 1867, to December 20, 1871.

The premier of Ontario is the head of government of Ontario. Under the Westminster system, the premier governs with the confidence of a majority the elected Legislative Assembly; as such, the premier typically sits as a member of Provincial Parliament (MPP) and leads the largest party or a coalition of parties. As first minister, the premier selects ministers to form the Executive Council, and serves as its chair. Constitutionally, the Crown exercises executive power on the advice of the Executive Council, which is collectively responsible to the legislature.