The road begins at the intersection of Route 28, overlapping Route 39. It leaves Route 39 next to the First Parish Church in downtown Harwich. After 1.3 miles (2.1 km), the road passes the Mid-Cape Highway at Exit 10 off the Mid-Cape Highway. The road crosses into Harwich just north of a pass between three ponds-Seymour Pond, Hinckley's Pond, and Long Pond, one and a half mile north of the Mid-Cape Highway. After about a half-mile, the Cape Cod Rail Trailbike path crosses over and follows alongside the road for another one and a half miles. The road crosses Route 137 after two miles (3 km). After less than a half-mile the road ends at Route 6A and Harwich Road. The entire length of the road is 6.8 miles (10.9 km).
Route 124 was formerly designated Route 24, prior to the completion of the initial stretch of the current Route 24 freeway in 1951. When the freeway was given the Route 24 designation, the existing Route 24 was renumbered to 124.