The route runs from Route 190 in the Somersville section of the Connecticut town of Somers to the town center of East Longmeadow, Massachusetts.

Route 186 begins at an intersection with Route 190 in the Somersville section of Somers and heads north for about 3.6 miles (5.8 km) to the Massachusetts state line.

The road continues across the Connecticut-Massachusetts state line as Prospect Street for another 2.4 miles (3.9 km) towards East Longmeadow center.

Route 186 is classified as an urban collector road and carries average daily volumes of 2,000 vehicles.

As part of the 1962 Route Reclassification Act, the southern alignment was returned to its original location.