US 6 once stood as America's longest numbered highway, stretching from Cape Cod to Long Beach, Calif.

At its birth in 1926, US 6 was laid out in two comparatively small parts: from Cape Cod to the Connecticut/New York state border at Danbury, and from Kingston, N.Y to Erie, Pa. In 1928, the Kingston end was moved to Danbury to unite the two segments.
Around 1930, the Roosevelt Highway Association lobbied for the extension of US 6 westward. It subsumed US 32 and 38 from Chicago to Denver, and continued along other routes to Long Beach, and gained the name "Roosevelt Highway."

Some parts of US 6 predate route numbering by a century or more. Main Street in Woodbury, part of US 6 now, was laid out along an Indian trail in the 1670s. Much of present-day US 6  in Connecticut is part of The Revolutionary Road, used by the American Continental Army in at least five campaigns, including a successful Rhode Island battle that enabled the French to land at Newport eventually join the fight for American independence.   This road is also noted for French Gen. Rochambeau's march in the same era.

In 1936, the Grand Army of the Republic (an association of Union veterans of the Civil War) proposed that US 6 be named the "Grand Army of the Republic Highway." In 1937, the New York and Connecticut state legislatures agreed, and since then other states have followed. As GAR membership was limited strictly to those who served in the Civil War, the road soon served as a memorial to all members.