Niagara County is in the extreme western part of New York State, just north of Buffalo and adjacent to Lake Ontario on its northern border and the Niagara River and Canada on its western border. The border results in a fair number of illegal immigration cases, though these are more common to the east where the border is across land rather than a lake or river.

The primary geographic feature of the county is Niagara Falls, which has eroded seven miles south over the past 12,000 years since the last Ice Age. The Niagara River and Niagara Falls, are in effect, the drainage ditch for four of the Great Lakes which constitute the largest supply of fresh water in the world. The water flows north from Lake Erie, then through the Niagara River, goes over Niagara Falls, and then on to Lake Ontario and the St. Lawrence River, eventually emptying into the North Atlantic Ocean. Today, tourists and visitors to the Falls only see half of the natural water flow over the Falls, since the other half has been taken for hydroelectric power purposes. Both the American and Canadian side of the Niagara River have massive electrical power plants.

The spectacular Niagara Gorge is the path Niagara Falls has taken over thousands of years as it continues to erode. Niagara Falls started at the Niagara Escarpment which cuts Niagara County in half in an East-West direction. North of the Escarpment lies the Lake Ontario plain, which is a fertile flatland that is used to grow grapes, apples, peaches and other fruits and vegetables.