The North Branch rises in New Ipswich, New Hampshire, at the outlet of Mountain Pond. 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.
The North Branch is 6.0 miles (9.7 km) long, 4.0 miles (6.4 km) of which are in New Hampshire, with 2.0 miles (3.2 km) in Massachusetts. 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).