Try the resources linked below for each of the following date ranges:
pre-1869: Try the Guide to Church Records for more information
1779 – 1858: Marriage Bonds, 1779-1858 - Upper & Lower Canada from the Library and Archives Canada
1786 – 1870: The District Marriage Registers Of Upper Canada/Canada West (no Midland District registers exist)
1858 – 1869: Ontario, Canada, County Marriage Registers, 1858-1869 (Ancesty Library Edition, limited access)
1869 – 1927: Ontario Marriages, 1869-1927 on FamilySearch
1928 – 1932: Marriage Records using the Archives of Ontario Microfilm Interloan Service.
1933 – 1936: Ontario, Canada, Marriages, 1826-1936 (Ancesty Library Edition, limited access)
1937: Closed for microfilming
1938 – present: Registrar General's Online Certificate Application (Service Ontario)
Resources
Books
County Marriage Registers Of Ontario, Canada, 1858-1869
Frontenac County (Ontario) Marriages
4 Volumes: Vol.1 (1869-1875), Vol.2 (1876-1880), Vol.3 (1881-1885), Vol.4 (1886-1890).
Marriage Bonds Of Ontario, 1803-1834
Marriage Notices of Ontario
Ontario Marriage Notices
The District Marriage Registers Of Upper Canada/Canada West
(no Midland District registers exist)
Databases
Births, Marriages and Deaths Recorded in Canada
Family events such as baptisms, marriages and burials are key elements of genealogy and family history research. From early times to the present, these events have been recorded in church records and in other official documents. Library and Archives Canada holds a small collection of such documents, some of which are indexed by name.
— from the Library and Archives Canada website
Canadian Birth, Death and Marriage Records on FamilySearch
Marriage Bonds, 1779-1858 - Upper & Lower Canada
Use the Search: Database link in the left menu to search the database.
Ontario Marriage Registrations, 1800-1927 on the Ontario Vital Statistics Project
Ontario Marriages, 1869-1927 on FamilySearch
Index to marriage records housed at Archives of Ontario in Toronto. Includes regular registrations, 1869-1927, and delayed registrations, 1892-1919.
Ontario, Canada, County Marriage Registers, 1858-1869
Ontario, Canada, Marriages, 1826-1936
Microfilms
Archives of Ontario Microfilm Interloan Service
Websites
District Marriage Registers & District Vital Records of Upper Canada/Canada West (Ontario) 1786 - 1870
An article from Global Genealogy explaining district marriage registers & district vital records of Upper Canada/Canada West, some of which are sold directly on the site.