Constantine Sidamon-Eristoff died on Monday in his home on the Upper East Side. He was 81. Connie was one of the Association’s greatest friends and supporters and will be greatly missed. The cause was esophageal cancer, his son Andrew said.

Prince Constantine Sidamon-Eristoff was born in Manhattan on June 28, 1930, and grew up both in the city and in Highland Falls, N.Y., along the Hudson River near West Point. A prince whose family nobility dates to the 15th century in the Eurasian kingdom of Georgia (now an independent republic that shares borders with Russia, Turkey and the Black Sea), Mr. Sidamon-Eristoff was a kind of supercitizen conservationist, performing innumerable decision-making roles in jobs dealing with the environment in and around New York City.

Constantine Sidemon-Eristoff had served as a member and Trustee of the C I Association since 1963, covering a span of 48 years. His heritage includes several of the Association’s founding families, the Tracys, Morgans, Bigelows, and Sidemon-Eristoffs. His family had sustained a long-standing friendship with Susan and Anna Warner over many years. Connie’s relatives and family have also been involved in the 1916 founding of the then-titled Martelaer’s Rock Association— now know as the Constitution Island Association.

Connie assumed his trusteeship in 1963, not only as a family obligation, but also as a personal commitment to the purpose and mission of the Association. He has contributed his knowledge and expertise to improving and revising the by-laws of the Association; and has been instrumental in obtaining the Absolute Charter which was awarded by the State University of New York Board of Regents in 1965. He has given invaluable advice and guidance on crucial committees such as: Finance: from 1980-1987 and as chair of that committee from 1987 to 1999; Fund Raising in 1986. He served as Nominations Committee member from 1990 to 1994 and as chairman from 1994 to 2002 and also as the chairman of the combined Personnel/Nominations committee from 2002 to 2011. He honorably and generously supported the Association’s purpose and mission, continuing his family’s involvement and friendship with the Warner family. Connie was given the Faith Herbert Award for distinguished service to the Constitution Island Association in 2008.

Young Connie went to the Millbrook School in Dutchess County, N.Y., and studied geological engineering at Princeton before serving in the Army in the Korean War and receiving a Bronze Star. After earning a law degree from Columbia in 1957, he worked as an executive assistant for John V. Lindsay, who was then a congressman, and helped organize his subsequent campaign for mayor.

His father, Prince Simon Sidamon-Eristoff, was an Imperial Russian Army officer during World War I and, in a brief period of Georgian independence, the chief of staff of the Georgian Army; he emigrated to the United States after the Bolsheviks invaded Georgia in 1921. (The hyphenated name, a Russianized version of the original Georgian, Sidamoni-Eristavi, is a family name plus a title. Eristavi, in Georgian, translates loosely as “head of the people,” Andrew Sidamon-Eristoff said.)

In the United States, Prince Simon married Anne Tracy, a descendant of John Bigelow, an American diplomat in the mid-19th century.

In addition to his son Andrew, Mr. Sidamon-Eristoff is survived by his wife, the former Anne Phipps, whom he married in 1957; a sister, Anne Sidamon-Eristoff; two other children, Simon and Elizabeth; and eight grandchildren.