Maine was granted statehood on March 15, 1820, as part of the Missouri Compromise.

Nine of the 16 counties had their borders defined while Maine was still part of Massachusetts, and hence are older than the state itself.

[1][page needed] Even after 1820, the exact location of the northern border of Maine was disputed with Britain, until the question was settled and the northern counties signed their final official form, the Webster–Ashburton Treaty, signed in 1842.

The most populous counties tend to be located in the southeastern portion of the state, along the Atlantic seaboard.

The largest counties in terms of land area are inland and further north.