 He was a member of the Metropolitan Club, the Alibi Club, the Knickerbocker Club, and the Meadowbrook Club on Long Island.William Eustis owned and raced Thoroughbred horses and in addition to riding polo ponies was a very capable amateur Thoroughbred rider.As part of the undercard for the May 29, 1897 Belmont Stakes won by Scottish Chieftain at Morris Park Racecourse, William Eustis rode Hawarden in the one mile Amateur Cup.