In the 1920s, State Highway 138, unrelated to today's Route 138, went from Unionville to Canton via today's Route 4, Route 179, Maple Ave and Dowd Ave (both now SR 565) to US 44.
At the same time, much of modern-day Route 138 was called SH 321.
The history of today's Route 138 starts outside Connecticut, as a route leading from Newport, R.I. to Milton, Mass., designated in the 1920s. In the early 1930s, neighboring New England states got together to work out how state highways could keep their numbers across state lines. Rhode Island must have discussed its plan to extend Route 138 westward; on Jan. 1, 1932, Connecticut designated its own portion of what would be a three-state route.
In 1932, Route 138 was commissioned as a 13.88-mile route between Route 93 (now Route 169) in Griswold, and the Rhode Island state line in Voluntown.
The route followed two short older alignments in Griswold:
- Pachaug section at Edmund Road: Goldstein Road, Browning Road, and a vanished segment east of Edmund Road
- at Route 201: Chinigo Road (formerly "Old Route 138")
... and one longer old alignment in Voluntown:
Rockville Road, today's Route 138 heading into Rhode Island, was not state maintained at the time. Route 165 itself ended in Voluntown and did not reach the state line.
In 1937, the state added Rockville Road as unsigned SR 420. In 1938, Route 138 was moved to Rockville Road, where it remains today, and Beach Pond Road became SR 420. In 1947, Route 165 was extended along SR 420 to the state line. This was likely done in cooperation with Rhode Island.
Around 1956, Route 138 was reconstructed in the vicinity of Edmond Road in the Pachaug section of Griswold; some houses were taken. The old segment was called SR 520 or 526 for a short time, and then turned over to the town in 1961. A short segment at Route 201 was realigned around the same time, and a short secret route (526 or 520) left over was also cancelled in 1961.
In 1961, the recommendations of the state committee to reclassify highways recommended adding a route from Route 97 in the Baltic section of Sprague to Route 169 in Lisbon Center. In 1963, this was added as a western extension of Route 138.
In 2003, there was discussion among officials whether the town should retain the old road near Route 201 or abandon it; the going sentiment was to turn it over to a private property owner who holds acreage on both sides.