Thornwood is a hamlet (unincorporated community), census-designated place (CDP), and postal designation (with zip code 10594) in the town of Mount Pleasant in Westchester County, New York. The population was 3,759 at the 2010 census.
In 1891, Lewis Smadback started a very successful development at what is now Thornwood. The village of Sherman Park was incorporated in 1906 and the name was changed to Hillside in 1909. The village was dissolved in 1914, when Thornwood took its current name and became a hamlet within the Town of Mount Pleasant. It is possible that Thornwood is a derivative of "Hawthorne's Woods," since Hawthorne is an adjacent hamlet. Another possibility is Thornwood is named after an area in Glasgow, Scotland.
Thornwood once had a large and thriving Westchester marble quarry near its heart, the intersection of Route 141 and Kensico Road (known as Four Corners). Dating back to 1845, the quarry supplied white marble which helped build St. Patrick's Cathedral (hence the name of Marble Avenue which runs through the hamlet). The quarry pit was filled in during the mid-1980s, and the Town Center shopping mall was constructed over it.