Wordorigins.org

The name Ontario was first applied to the lake and only later to the territory comprising the Canadian province. The word is a French spelling of the Mohawk õtaríyo, which means beautiful lake, õtar (lake) + iyo (beautiful).

French usage for the name of the lake is found as early as 1641. By the end of the 17th century it was appearing in English texts as well. An Ontario County was organized in 1792 (not the current one). The current Ontario County was formed in 1849. The province was officially named Ontario in 1867.

(Sources: Dictionary of Canadian Place Names; Oxford English Dictionary, 3rd Edition)