In Canada, a ministry is formed when a new prime minister is appointed and dissolved when that individual leaves office.
The one exception occurred in 1917, when the incumbent prime minister, Sir Robert Borden, formed a new national unity government (the 10th Canadian Ministry) as a wartime coalition composed primarily of members of his own Conservative Party with some individual Liberal Party members of parliament.
In contrast to various other Commonwealth realms (such as Australia and the United Kingdom) where a "new" ministry is considered to have been formed after every general election regardless of the winner, elections in Canada do not cause dissolution of the ministry unless they result in the government's defeat.
As such, the current 29th Ministry, chaired by Prime Minister Justin Trudeau, began governing shortly before the opening of the 42nd Parliament in 2015.
William Lyon Mackenzie King led three ministries—the 12th, 14th, and 16th—the most for any Canadian prime minister.