Map - St. Lawrence County, New York (St. Lawrence County)

St. Lawrence County is a county in the U.S. state of New York. As of the 2020 census, the population was 108,505. The county seat is Canton. The county is named for the Saint Lawrence River, which in turn was named for the Christian saint Lawrence of Rome, on whose feast day the river was visited by French explorer Jacques Cartier.

St. Lawrence County comprises the Ogdensburg-Massena, NY Micropolitan Statistical Area and is New York's largest county by area.

When counties were established in the Province of New York in 1683, the present St. Lawrence County was part of Albany County. This was an enormous territory, including the northern part of New York State as well as all of the present State of Vermont and, in theory, extending westward to the Pacific Ocean. The county was reduced in size on July 3, 1766, by the creation of Cumberland County, and further on March 16, 1770, by the creation of Gloucester County, both containing territory now in Vermont.

On March 12, 1772, what was left of Albany County was split into three parts, one remaining under the name Albany County. The other two were called Tryon County (later renamed Montgomery County) and Charlotte County (later renamed Washington County). Tryon County contained the western portion (and, since no western boundary was specified, theoretically extended west to the Pacific). The eastern boundary of Tryon County was approximately five miles west of the present city of Schenectady, and the county included the western part of the Adirondack Mountains and the area west of the West Branch of the Delaware River. The area then designated as Tryon County includes what are now 37 counties of New York State. The county was named for William Tryon, colonial governor of New York. Charlotte County contained the eastern portion of Albany County.

In 1784, following the peace treaty that ended the American Revolutionary War, the name "Charlotte County" was changed to Washington County to honor George Washington, the American Revolutionary War general and later President of the United States of America. Tryon County was changed to Montgomery County to honor the general, Richard Montgomery, who had captured several places in Canada and died trying to capture the city of Quebec; it replaced the name of the hated British governor.

In 1788, Clinton County was split off from Washington County. This was a much larger area than the present Clinton County, including part of what would later become St. Lawrence County, as well as several other counties or county parts of the present New York State.

In 1789, the size of Montgomery County was reduced by the splitting off of Ontario County from Montgomery. The actual area split off from Montgomery County was much larger than the present county, also including the present Allegany, Cattaraugus, Chautauqua, Erie, Genesee, Livingston, Monroe, Niagara, Orleans, Steuben, Wyoming, Yates, and part of Schuyler and Wayne Counties.

St. Lawrence County is part of Macomb's Purchase of 1791.

In 1791, Herkimer County was one of three counties split off from Montgomery (the other two being Otsego, and Tioga County). This was much larger than the present county, however, and was reduced by a number of subsequent splits. The first was the splitting off in 1794 of Onondaga County. This county was larger than the current Onondaga County, including the present Cayuga, Cortland, and part of Oswego Counties. This was followed by the splitting off in 1798 from Herkimer County of two portions: one, Oneida County, was larger than the current Oneida County, including the present Jefferson, Lewis, and part of Oswego Counties; another portion, together with a portion of Tioga County, was taken to form Chenango County.

In 1799, Clinton County was reduced in size by the splitting off of Essex County from Clinton County.