This was as named by early French explorer Jacques Cartier for the Christian saint Lawrence of Rome, as he visited the river on the saint's feast day.

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 named for William Tryon, colonial governor of New York.

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.

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.

This was much larger than the present county, however, and was reduced by a number of subsequent splits.

The selection of Canton as the county seat was a compromise by the state legislature to end competition between factions supporting Ogdensburg and Potsdam for the county seat.

[3] On September 5, 1944, a 5.8 magnitude earthquake centered in Massena struck the county.

This waterway allows ships and vessels to pass through the St. Lawrence River and on to the Great Lakes.

Ogdensburg Harbor Light is listed in the National Register of Historic Places, and in the National Register of Historic Places listings in St. Lawrence County, New York As of the census[10] of 2000, there were 113,931 people, 40,506 households, and 26,936 families residing in the county.

26.50% of all households were made up of individuals, and 11.20% had someone living alone who was 65 years of age or older.

From 1992 through the 2012 election, St. Lawrence County swung Democratic, posting double-digit victories for Democratic candidates, most notably in 1996 when Bill Clinton won the county by 28-point margin over Bob Dole.

In 2020, it was one of only a few counties in Upstate New York where Trump improved his margin, this time carrying it by over 10 points.