Route 275's number arises from the short-lived, unsigned SR 575 along the same route. State records imply the designation was first applied in 1957, but cancelled later that year.

In 1960, the state Committee to Reclassify All Public Roads recommended adding Stonehouse Road in Coventry and South Eagleville Road in Mansfield to the state highway system. In 1962, SR 575 was designated along those roads. In 1963, SR 575 was promoted to signed Route 275.

At first, local and state agencies were concerned with safety improvements to the road; into the 1970s, there were nascent plans to extend the highway west to a proposed interchange with I-84 near Andover.

In July 1966, Mansfield citizens met with the state Highway Department, asking for the removal of several safety hazards, including blind intersections, steep banks and grades, and sharp curves. The next year, a public hearing was held to discuss growing traffic, mostly to and from the University of Connecticut (aka UConn).

In June 1967, a bill headed to the state house included $1 million to improve Route 275 from Route 32 to Old Eagleville Road. That bill failed, but a substitute bill funded a $1 million bridge over the railroad and Willimantic River, replacing old one-lane bridges across each. At some point this bridge was completed, and Route 275 was realigned in the area, but I don't have a date for it.

Extension to US 6 (I-84) freeway proposed

In 1969, the state proposed extending Route 275 west to an interchange with I-84 (also proposed, and to this date still unbuilt) near Andover. The interchange would be built near or at Parker Bridge Road. The extended Route 275 would cross Route 31 south of their present intersection. Local residents and officials asked that proposed new access roads at UConn be built first, as otherwise the extended Route 275 would dump more traffic on Route 195.

In December 1970, a local petition opposing the Route 275 extension was circulated. However, ConnDOT replied that there were no plans to extend the highway at that time. The following year, Mansfield came out in favor of a north-south alignment for Route 275; details are missing, but the road would probably connect to the upcoming section of I-84 (now US 6) bypassing Willimantic.

The I-84/US 6 freeway from Bolton to Columbia continued to be an issue throughout the 1970s, as well as the interchange at Parker Bridge Road. In January 1979, local officials favored the interchange, but expressed concern about UConn traffic there. ConnDOT stated again that there were no plans to improve Route 275.

Eagleville bypass proposed

In June 1971, a southern bypass of Eagleville for Route 275 was proposed. The two-lane road would cross Route 32 on new alignment.

As of 2001, the state still planned a realignment between Separatist Road and Route 32, along with improvements from there to Route 31. The scope of the work would depend on whether the US 6 expressway is completed.