Millwood is a hamlet and census-designated place located in the town of New Castle, New York, United States in Westchester County. It was originally settled as Sarlesville. The area now known as Millwood appears on 19th century maps as Merritt's Corners and Rockdale Mills. As of the 2000 census, the community had a population of 1,210.

New Castle is a town in Westchester County, New York, United States. The population was 18,311 in the 2020 United States census, an increase over 17,569 at the 2010 census. It includes the named hamlets of Chappaqua and Millwood, but residents and businesses in the Town of New Castle can also have a designated city address of Ossining, or Millwood as well as Chappaqua or even Mt. Kisco.