The route begins in the northwestern portion of the Connecticut state capital of Hartford and ends in the rural town of Granville.

In Massachusetts, the route is not a state highway and is maintained by the town of Granville.

North of town, it turns slightly northeast, and meets Route 187 to form a four-lane freeway.

In Simsbury, it enters the Tariffville section of town, where it becomes a two-lane surface road and meets the eastern end of Route 315 before crossing the Farmington River into East Granby.

In 1960, five years after two of the bridges were washed out in a flood, Route 9 was relocated to stay on the south side of the river along a newly built expressway.