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, passing Beaver Brook Falls Wayside, to its headwaters at Stewartstown Hollow in the town of Stewartstown, where the route follows Hollow Road.

A New Hampshire historical marker there (number 47) notes the nearby gravesite of Metallak, "The Lone Indian of the Magalloway".

North of there, the highway descends to and crosses the Connecticut River into the town of Pittsburg.