The Town of North Castle comprises approximately 26 square miles and is situated at the narrow waist of Westchester County where the corner of the State of Connecticut extends west toward the Kensico Reservoir. The bulk of the Town's land area lies north of this corner, but the most densely populated part of the Town lies to the south. The Kensico Reservoir separates these two parts of North Castle.

The Town has been divided into three distinct geographic areas:
North White Plains, Armonk, and the Eastern District (the hamlet area in the Eastern District is referred to as Banksville). North White Plains is the most urban portion of the Town.  The seat of Town government is located in Armonk.  The Eastern District retains its low-density residential character.

A number of highways link the Town with neighboring communities and other parts of the New York Metropolitan Region. Route 22 connects the Town with White Plains and Bedford. Route 128 links Armonk with Mount Kisco and Bedford-Banksville Road joins Bedford and Greenwich. King Street (Route 120) provides a connection to Port Chester, Rye Brook, and Chappaqua. Interstate 684 is a major regional north-south route, which connects to other expressways, and parkways, providing access to all parts of the New York Region and beyond. The Bronx River and Merritt Parkways also serve the Town, although the latter does not pass through the Town.