 In 1905, De Peyster purchased first baronial mansion of Sir William Johnson, known since 1755 as Fort Johnson, and donated it to the Montgomery County Historical Society, of Amsterdam, New York.In 1901, he donated several thousand books and maps to the Smithsonian Institution, along with a Moorish Yataghan he collected on his travels in 1851.De Peyster's biographer devotes six chapters to his benefactions, but does not mention his ethnological collections.