In pre-colonial times, the land now occupied by Aroostook County was inhabited by the Micmacs and the Maliseets, bands of which still reside in the area.  French explorers, led by Samuel de Champlain and Sieur de Monts, first visited the area in 1604, but the area was not settled until much later.

The first settlers arrived in the Aroostook River area during a border dispute between the United States and Great Britain. They settled on property claimed by both sides.  When Maine was admitted to the Union in 1820, a large part of what was to become Aroostook County still belonged to Massachusetts.

In 1837, Maine became the only state to declare war unilaterally when its legislature declared war against Canada and dispatched an army of 200 to what is now Aroostook County. Eventually the U.S. Congress appropriated funds and raised 10,000 militia to defend its borders against the British. The Webster-Ashburton treaty, adopted in 1842, settled the dispute without bloodshed and fixed the border between Maine and New Brunswick.