Route 27 was commissioned in 1949, as a 9.84-mile route leading from Route 84 (now Route 184) in Groton to Route 12 in Preston. It absorbed all of SR 677 and part of SR 664. Its route, from south to north:

• Col. Ledyard Highway, from Route 184 to Center Groton Road (taking over SR 677)
• Col. Ledyard Highway, continuing to Poquetanuck Road. This is now part of Route 117.
• Poquetanuck Road, west to Route 12. This is now part of Route 2A.

At the time, Route 27 intersected no other signed state highway between its endpoints. None of the original route exists as part of Route 27 today.

Today's Route 27 was originally called Route 169 (no relation to the modern route 169), and differed slightly from today's route: the southernmost leg followed Willow Street to US 1 instead of Dennison Avenue.

In 1951, Route 27 was extended south, including an overlap with Route 84, to incorporate all of Route 169. In 1960, Route 27 was modified to use today's alignment along Dennison Avenue.

In 1963, Route 27 was removed north of Route 184, leaving the 3.21-mile segment we see today. The Poquetanuck Road segment became SR 640, and later Route 2A. About half the Col. Ledyard Highway segment became part of Route 117. Some of the remainder became SR 641, which was turned over to the city of Groton in 1966.

Extend (renumber) northward along Route 201 to Route 12 in Griswold.

Route 27 Ends (Photos; Doug Kerr at state-ends.com)