It parallels the Atlantic Ocean from New Hampshire north through Portland, Brunswick, and Belfast to Calais, and then the St. Croix River and the rest of the Canada–United States border via Houlton to Fort Kent.

[3] The route's most northerly geographic point is at its intersection with the Edmundston–Madawaska Bridge in Madawaska, across the Saint John River from Edmundston, New Brunswick, Canada.

US 1 south of Calais was initially part of the Atlantic Highway and became Route 1 when the New England road marking system was established in 1922.

By 1925, however, Maine had transferred the Route 24 designation to a completely new alignment on the eastern edge of the state, running from Calais to Madawaska at a border crossing with Edmundston, New Brunswick.

The portion between Portland and Brunswick was rebuilt, mainly as a four-lane divided highway, in the 1950s, and later absorbed into I-95 (now I-295).