The 2,300-acre (9.3 km2)[1] (or 2,500-acre (10 km2))[2][3] estate was purchased in stages by assembling five farms,[1] beginning in 1889,[4] by German-born acetylene gas mogul Charles F. Dieterich (1836–1927),[3] a founder of Union Carbide.
The Castalia Foundation also hosted weekend retreats on the estate where people paid to undergo the psychedelic experience without drugs, through meditation, yoga, and group therapy sessions.
[12] Nina Graboi described Millbrook as "a cross between a country club, a madhouse, a research institute, a monastery, and a Fellini movie set.
'"[13] During Leary's residence at the mansion (1963–1968) the culture and ambiance there evolved from scholarly research into psychedelics to a more party-oriented atmosphere, exacerbated by an increasing stream of visitors, some youthful and of the hippie persuasion.
But after about two decades of effort by the late architectural historian John Foreman, whose tenancy was conditional on his undertaking its restoration and preservation, the house is (as of 2016[update]) habitable although not modernized.