List of Ontario by-elections

The list of Ontario by-elections includes every by-election held in the Canadian province of Ontario. By-elections occur whenever there is a vacancy in the Legislative Assembly, although an imminent general election may allow the vacancy to remain until the dissolution of parliament.

Causes
A by-election occurs whenever there is a vacancy in the Ontario legislature. Vacancies can occur for the following reasons:


 * Death of a member.
 * Resignation of a member.
 * Voided results
 * Expulsion from the legislature.
 * Ineligibility to sit.

When there is a vacancy, a by-election must be called within six months. Under amendments to the Election Act approved in 2016, a by-election is no longer required when a vacancy occurs in the 12 months leading up to a fixed general election date.

Ministerial by-elections
The list includes ministerial by-elections which occurred due to the requirement that incumbent members recontest their seats upon being appointed to Cabinet. These by-elections were almost always uncontested. This requirement was amended in 1926 to exempt ministers appointed within three months after a general election. In 1941 it was abolished completely.

30th Legislative Assembly of Ontario 1975–1977
no by-elections

25th Legislative Assembly of Ontario 1955–1959
† Won by acclamation

20th Legislative Assembly of Ontario 1937–1943
† Won by acclamation

* Though nominally an "Independent-Liberal", MacBride was a supporter of the Liberal government and was a cabinet minister at the time of his death

19th Legislative Assembly of Ontario 1934–1937
† Won by acclamation

18th Legislative Assembly of Ontario 1929–1934
† Won by acclamation

17th Legislative Assembly of Ontario 1926–1929
† Won by acclamation

16th Legislative Assembly of Ontario 1923–1926
† Won by acclamation

15th Legislative Assembly of Ontario 1919–1923
† Won by acclamation

14th Legislative Assembly of Ontario 1914–1919
† Won by acclamation

13th Legislative Assembly of Ontario 1911–1914
† Won by acclamation

12th Legislative Assembly of Ontario 1908–1911
† Won by acclamation

11th Legislative Assembly of Ontario 1905–1908
† Won by acclamation

9th Legislative Assembly of Ontario 1898–1902
† Won by acclamation

8th Legislative Assembly of Ontario 1894–1898
† Won by acclamation

7th Legislative Assembly of Ontario 1890–1894
† Won by acclamation

6th Legislative Assembly of Ontario 1886–1890
† Won by acclamation

5th Legislative Assembly of Ontario 1883–1886
† Won by acclamation

4th Legislative Assembly of Ontario 1879–1883
† Won by acclamation

* McAllister was a supporter of Mowat's government

3rd Legislative Assembly of Ontario 1875–1879
† Won by acclamation

2nd Legislative Assembly of Ontario 1871–1874
† Won by acclamation