Its eastern terminus is at the Canada-US border in Vanceboro, where it connects to New Brunswick Route 4.

SR 6 is the only highway in Maine to terminate at the Canadian border at both ends.

With a length of 207.23 miles (333.50 km), it is the third-longest state highway in Maine.

The only section not shared with another route is from its junction with US 2 in Lincoln east to the New Brunswick border, covering 61.87 miles (99.57 km).

In 1965, SR 6 was extended west to its modern terminus at the Quebec border by overlapping it entirely with existing highways.