It is a tributary of the Millers River, which flows west to the Connecticut River, which in turn flows south to Long Island Sound, an arm of the Atlantic Ocean.

The North Branch rises in New Ipswich, New Hampshire, at the outlet of Mountain Pond.

[1] It flows west through Island Pond into Rindge, and passes the villages of East Rindge and Converseville to Lake Monomonac.

From the lake's outlet in Massachusetts, the North Branch flows south parallel to U.S. Route 202, joining the Millers River at Whitney Pond in Winchendon.

If the channel length of 2.9 miles (4.7 km) through Lake Monomonac were included, the total length would be 8.9 miles (14.3 km).