Route 218 was commissioned in 1935. Portions of the route were already unsigned state highways: SR 910 in Bloomfield and SR 920 in West Hartford. The original Route 218 followed this alignment:

• Simsbury Rd (today's Route 185), from Route 9 (now Route 189) to Hall Blvd.
• Hall Blvd. and Cottage Grove Rd. (today's Route 218) to Walsh St.
• Walsh St. (former Cottage Grove Rd./218) to Route 184 (now Route 187)

Routes 218 and 185 had an uncommon "bump" junction where the roads intersected but did not cross.

1947: Trade offer for Route 187 declined

In 1947, Bloomfield officials submitted a bill to have the state accept Blue Hills Avenue Extension, a portion of today's Route 187 north of Park Avenue, into the state highway system. Gov. McConaughy vetoed the bill, saying the two population centers served by the road were already served by parallel trunk routes (such as Route 9, which is now Route 189).

Gov. McConaughy also noted that the state had offered to turn over the Cottage Grove Road portion of Route 218 to the town in return for the Blue Hills segment, but that offer had been declined.

In a general note attached to his veto, he criticized the recent practice of the Legislature to prod the Highway Commissioner into incorporating more and more local roads into the state system. He concluded: "I believe the interests of the whole state should take precedence over the desires of the local community, as much as I would like to sign each bill that improves local roads."

CG Campus and Route 218 upgrade

In the mid-1950s, the state highway plan included a four-lane expressway along Route 218, from Hall Blvd. all the way to Wilson, to connect with the new Bissell Bridge. This was not implemented, but in 1957, Route 218 was widened to a four-lane boulevard for 0.5 miles between Hall Blvd. and Route 189, around the opening of the Connecticut General Life Insurance office campus.

The campus itself was the site of a landmark conference, titled "The New Highways: Challenge to the Metropolitan Region," that marked an early pushback by urban planners against highway expansion.

Pundits also decried the campus, set on 450 acres of rural land, as another milestone in the exodus of industry and commerce from the city to the suburb. By 2000, however, when some buildings were to be demolished, sentiment had taken an unexpected turn: the National Trust for Historic Preservation had named one of the buildings among the 11 most endangered historic places in America.

On July 12, 1956, at 12:01 am, the half-mile of Route 218 corresponding to the first four-lane divided section was added to the state trunk line system. (This event made the Hartford Courant – while other changes like the creation of Route 287 did not.) In November 1956, detours were set up around the construction area while Route 218 was widened.

1963: Routes 185 and 218 are untangled

In 1963, Routes 218 and 185 were straightened out by swapping their southern parts, which sounds more salacious than it really is. In the 1980s, spot improvements were done at various intersections, including a reroute at Blue Hills Avenue to form a four-way intersection with the Putnam Highway in 1982.

The aftermath of cancelled I-291 (and built I-291)

A glance at Route 218 on a map shows a road passing north, then east, around Hartford, like a quadrant of a poor man's beltway. The comparison is apt, as an interstate beltway (I-291) was planned, and later cancelled, along a corridor quite close to Route 218.

Even before I-291 existed as a concept, plans for circumferential Hartford highways included the Cottage Grove Road corridor.

In September 1973, when the I-291 proposal was still alive (though controversial), the Bloomfield Town Council asked the Town Manager to develop a lower-profile replacement for the city. In November 1973, officials released plans for a new four-lane divided highway (not a freeway) to relieve east-west traffic. The new road, with 120-foot right of way (including 20 feet for future mass transit) would lie south of present-day Route 218, leading from Route 185 in Bloomfield to Wolcott Avenue in Windsor.

Proponents and opponents of the new highway attached various outcomes to its completion:

• relieve traffic as the town waits for I-291
• help defeat I-291 by providing a suitable alternative
• help Cottage Grove Road return to residential street
• work in concert with a future I-291 – its layout was designed not to interfere

In 1974, shortly after I-291 was cancelled, the state studied relocating Route 218 as a four-lane divided highway with access partially controlled, on an alignment similar to the Bloomfield plan.

The relocated highway would have continued east from the Route 218/185 intersection, passing through the Connecticut General campus, and paralleling Cottage Grove Road to the south until nearly reaching Blue Hills Avenue. An undivided four-lane road would have continued, carrying Route 218 to exit 35 at I-91.

This was not implemented; around 1982, however, Route 218 was widened at the Route 185 intersection; and on Nov 13, 1981 it was rerouted along a new four-lane divided Mount St. Benedict connector to meet Route 187 across from Wolcott St.

I-291 was completed from I-91 easterly to I-84 in Manchester in 1994. The state prepared for increased traffic to Bloomfield by widening Route 218 to four lanes divided all the way from Hall Blvd. to Route 187 circa 1992, and extending Route 218 as a four-lane undivided road to Route 159 on Nov. 1, 1992. The extended Route 218 was given separate ramps to and from I-291, and a full interchange with I-91.