Route 85 was commissioned in 1932 from the old SH 102 (New London to Colchester) and SH 366 (Hebron to Bolton). The portion south of Colchester follows the 19th-century New London Turnpike road.

Route 85's north end has flipped a few times. It originally entered Manchester along Camp Meeting Road (SR 534) to end at Route 83 (or somewhere short of there). According to state highway maps, it was shifted north to meet US 6 and 44 in Bolton (near today's terminus, if not at it). In 1954, it was shifted back to Camp Meeting Road.

On Mar. 29, 1963, it was moved for the last time to its current route. It took over former SR 807 (Clark Road and Bolton Center Road) to US 6/44, and Camp Meeting Road became SR 534.

Even after that, the northbound Route 85 mainline followed SR 534 toward Manchester; to continue on 85 was a right turn. Around 2000 this intersection was rebuilt into a conventional T-intersection.

A 1931 study by the Traffic Section Department of Motor Vehicles called for a traffic circle at the Salem intersection with Route 82. A roundabout was completed there a few years later, in 2012.

Proposed Freeway becomes Route 11

Statewide plans in the 1950s included a freeway connecting Hartford and New London, using Routes 2 and 85. The Route 85 freeway would extend from Route 2 in Colchester to US 1 in Waterford.

Part of this road was built in 1972, from Route 2 in Colchester to Route 82 in Salem. In July 1971, however, the DOT had announced that in order to minimize motorist confusion and signing costs, the old Route 85 would keep its number. The new Route freeway opened as Route 11, and there's much more history on the Route 11 page.

In 1956, Route 85 was reconstructed in the Gilead area of Hebron. Salt Box Road and Prentice Hill Road are old alignments of the route.

In 1984, Route 85 was widened to four lanes with a median near the newly opened Crystal Mall in Waterford.