Many counties, including Pike County Kentucky, St Louis County Minnesota, and St Lawrence County New York, have at one time or another claimed to be “The largest county east of the Mississippi”.

However, according to the US Census Bureau, Aroostook County, Maine, which lies along the US/Canada border, takes first place honors with a total land area of 6,671 sq mi (17,280 km sq). Water area adds another 156 square miles.

The northernmost village in the county, Estcourt Station, has the distinction of being the northernmost community in the contiguous United States. Aroostook County is known for potato crops and its Acadian culture. Many residents are bilingual, speaking both English and Acadian French.

According to the Census Bureau:

St Louis County Minnesota, the largest county in that state, incorporates 6,247 sq mi of land (16,180 km sq), making it the second largest. As might be expected in Minnesota, which is known as the “Land of 1000 Lakes” water area adds a substantial 612 sq mi.

By comparison, St Lawrence County, NY (named for the St Lawrence River) has a land area of 2,680 sq mi (6,900 km sq) with an additional 141 square miles of water.

Pike County is the easternmost county in the Commonwealth of Kentucky and has a land area of 787 sq mi (2,040 km sq) and a water area of 1.8 sq miles. While being nowhere near the largest in land area, Pike County was second largest in Kentucky in coal production in 2013.

More Info: en.m.wikipedia.org