The county was first surveyed in 1792 as one of the nineteen counties created by Sir John Graves Simcoe in preparation for the United Empire Loyalists to settle here.
This county was home to several townships as well as the city of Brockville.
The original county was constituted in 1792, and was united with Frontenac County as an electoral district for the new Legislative Assembly of Upper Canada: ... which county is to bounded on the east by the westernmost line of the county of Grenvill, on the south by the river St. Lawrence, and on the west by the easternmost boundary line of the late township of Pittsburgh, running north until it intersects the Ottawa or Grand river, thence descending the said river until it meets the northwesternmost boundary of the county of Grenvill.
[2] In 1798, the Parliament of Upper Canada withdrew parts of Leeds and Grenville to form Carleton County and the three counties together were constituted as Johnstown District, effective at the beginning of 1800.
In 1838, the parts of the townships of Burgess and Elmsley north of the Rideau Canal were withdrawn from Leeds and transferred to Lanark County.