William Corcoran Eustis (July 20, 1862 – November 24, 1921) was a captain in the United States Army and the personal assistant to General John J. Pershing during World War I.

[7] During the first World War, William was a personal secretary to General John J. Pershing, achieving the rank of captain.

[6] Together Edith and William were the parents of five children, including:[7] He laid the cornerstone for the Corcoran Gallery of Art on May 10, 1894, which his grandfather funded.

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.

[6] He was interred at Oak Hill Cemetery in Washington, D.C.[20] Eustis owned and restored Oatlands Plantation in Leesburg, Virginia, until it was donated to the National Trust for Historic Preservation following his widow's death in 1964.