The CDP is bordered to the northwest by the city of Keene and to the west by the town of Swanzey.

The eastern border of the CDP follows NH 101 and Roxbury Road until north of Minnewawa Brook, and the northern border strikes west to the Keene line near Heath Road.

[3] Route 101 (Marlborough's Main Street) leads east 15 miles (24 km) to Peterborough and west 5 miles (8 km) to its terminus at Route 9 on the western side of Keene.

According to the U.S. Census Bureau, the Marlborough CDP has a total area of 1.7 square miles (4.4 km2), all of it recorded as land.

25.6% of all households were made up of individuals, and 11.4% were someone living alone who was 65 years of age or older.