Armonk is a hamlet and census-designated place in the town of North Castle, New York located in Westchester County. As of the 2010 census, Armonk's CDP population is 4,330 and it has a total area of 6.1 square miles, of which 6.0 square miles is land and 0.077 square miles, or 1.54�percent, is water. Armonk is well known as being the headquarters of IBM.