The southern terminus of Mammoth Road is in the center of Lowell at the junction with Pawtucket Street on the south bank of the Merrimack River, near the campus of University of Massachusetts Lowell. The road crosses over the river on the School Street Bridge and runs north toward New Hampshire. It passes through the town of Dracut and at mile 3.9 it crosses the state line. In New Hampshire the road becomes New Hampshire Route 128 in the town of Pelham, Hillsborough County. At mile 10.3 Mammoth Road crosses the county line into Rockingham County and into the town of Windham. At mile 11.7 the road intersects with Route 111, an east–west highway. At mile 14.9 Mammoth Road crosses Route 102 in the town of Londonderry. At mile 18.8 Mammoth road crosses Litchfield Road/Stonehenge Road and is the dividing line between the two named roads. Mammoth Road merges with Route 28/Rockingham Road at mile 20, heading north into Manchester for a short stretch. This point is the northern terminus of Route 128. Rockingham Road continues north for 1.7 miles, while Mammoth Road veers to the left and parallels Rockingham Road. At the northern end of this stretch, Mammoth Road once again connects with Route 28 until Route 28A. At mile 21.7, Mammoth Road follows Route 28A and Route 28 becomes South Willow Street, heading north-northwest into downtown Manchester. Here the road is South Mammoth Road. At mile 23.5 Mammoth Road/Route 28A crosses Interstate 293/Route 101. At mile 24.6 South Mammoth Road intersects with Huse Road and Cilley Road. Here South Mammoth Road drops the "South" and returns to Mammoth Road. At mile 26.2 the road crosses Bridge Street, a major east–west thoroughfare in Manchester. At mile 27.3 Mammoth Road merges with Smyth Road coming from the southwest and then crosses under Interstate 93. Mammoth Road and Smyth Road are merged for only 0.2 mile and then Smyth Road breaks off and continues east. After another 0.2 mile, at mile 27.7, Mammoth Road crosses the county line into Merrimack County and the town of Hooksett. The road is still called Mammoth Road in Hooksett. Though originally slated to continue its northward track to Concord, at mile 29.3 Mammoth Road ends at the junction with US-3/Route 28.
