Due to the reconfiguration of tunnel interchanges brought on by the completion of the Big Dig, Route 1A is discontinuous in the downtown Boston area.

It is a limited access highway through Revere, passing Suffolk Downs and the Wonderland Greyhound Park (which is across the street from the outer terminus of the MBTA's Blue Line), before running through the isolated Point of Pines neighborhood and exiting Revere over the General Edwards Bridge into Lynn.

At the intersection with Derby Street, notable for the large fire station, the route turns right.

It follows two of the main streets of downtown Beverly, before heading north, passing through Wenham and Hamilton (through this section the road is occasionally signed as a US Route) before becoming the main road through the town of Ipswich, crossing the historic Choate Bridge over the Ipswich River.

In Newburyport, Massachusetts, it merges with Route 1, signed with its parent highway over the Merrimack River and into the town center of Salisbury.

[4] In 1933, another, disconnected portion of Route 1A appeared between Attleboro and the Rhode Island state line.

As a result of that, Route 1A was extended south towards the Sumner/Callahan Tunnels and then north along Central Artery towards US 1 (Storrow Drive).

[10] In 1989, US 1 was rerouted from Storrow Drive onto I-93's Southeast Expressway, causing "signed" Route 1A to get truncated.