thumb|right|Merrickville, Rideau River c.1838

Grenville County is a former county in the Canadian province of Ontario. It fronted on the north shore of the Saint Lawrence River, between the towns of Kingston and Cornwall. The county was created in 1792, and named in honour of William Grenville, 1st Baron Grenville, who was the British Secretary of State responsible for the colonies in 1790. It consisted of five townships, which were settled primarily by United Empire Loyalists in the late 1700s after the Revolutionary War. Prior to being settled by Europeans, the area was home to many generations of native cultures. Grenville County merged with Leeds County in 1850 to create Leeds and Grenville County. The French briefly occupied this area at present-day Johnstown, in what was to become Edwardsburgh township, and at Pointe au Baril (present day Maitland) in what would be Augusta township. These French settlements date back to 1670 and 1759 respectively.

In 1798, the Parliament of Upper Canada withdrew parts of Grenville and Leeds to form Carleton County and the three counties together were constituted as Johnstown District, effective at the beginning of 1800. Grenville consisted of the following townships:

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|+ Organization of Grenville County (1800)

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The townships of

  • Edwardsburg
  • Augusta
  • Wolford
  • Oxford on the Rideau
  • Marlborough
  • Montague
  • North Gower
  • South Gower

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In 1838, the townships of Marlborough and North Gower were transferred to Carleton County, and the township of Montague was transferred to Lanark County.

When the Johnstown District was abolished in 1850, Grenville County was united with Leeds County to form the United Counties of Leeds and Grenville.

Original townships

thumb|left|[[Battle of the Windmill in New Wexford, Edwardsburgh, 1838]]

The five remaining townships at 1838 were Augusta, Edwardsburgh, Oxford-on-Rideau, South Gower, and Wolford.

  • Augusta township (still exists), covers an area of . It was first surveyed in 1783, and was named in honour of Princess Augusta Sophia, second daughter and sixth child of George III.