The Greentree Estate is a 400-acre property purchased  in 1904 by Payne Whitney for his bride, Helen Julia May, in 1904. Later, John Jay Whitney and his second wife Betsey, occupied the main house, where Mrs. Whitney remained in residence there until her death in 1998.

The original estate, in fact, is a consortium of five farms belonging to families Mitchell, Ketchum, Brinkerhoff, Skidmore, and Schenck, and was later divided as gifts of land made to several organizations. The Unitarian Universalist Congregation at Shelter Rock acquired property adjacent to Shelter Rock Road and has since constructed a large worship hall on the site of the former apple orchard. An honorary member of the Manhasset Lakeville Fire Department, John Jay Whitney donated the land along Community Drive East for the site of the Manhasset Company Two fire house, and of the Nassau County Police Department 6th, Precinct. Today, 408 acres, including the family home, are under the jurisdiction of the Greentree Foundation, run as a center dedicated to human rights issues.

From 1947 to 1977, radio and TV talk show pioneers John Reagan McCrary, his wife, model Jinx Falkenburg, and their sons lived on the Whitney estate.

In 2007, the estate hosted the Manhasset Negotiations, a week of domestic deliberations between Morocco and the Polisario Front, as part of UN-led meetings on the future of the Western Sahara.