NH 145 begins at US 3 (Main Street) just north of the U.S. Highway's bridge across the Mohawk River in the town center of Colebrook. The two-lane highway heads northeast out of the town center along Park Street. NH 145 follows Beaver Brook and its North Branch to its headwaters at Stewartstown Hollow in the town of Stewartstown, where the route follows Hollow Road. The highway descends to Cedar Brook and crosses Bishop Brook before ascending out of that valley and entering the town of Clarksville. NH 145 crosses Pond Brook and passes through the village of Clarksville. North of there, the highway descends to and crosses the Connecticut River into the town of Pittsburg. There, NH 145 follows Mill Street a short distance to its northern terminus at US 3 (Main Street).