His father, William Richards Castle, served King David Kalākaua as attorney general and later as Hawaiian Minister to the United States, where he was an active proponent of annexation.

William Richards Castle Jr. graduated from Punahou School and then Harvard College in 1900, where he was a founding member of the Fox Club.

During World War I he opened an American Red Cross bureau in Washington, DC, to assist in reuniting families and locating U.S. citizens missing overseas.

When he fixed his friendly eyes upon a minister or ambassador the envoy often would completely forget diplomacy and pour out his heart.

But behind the friendly eyes lurked a razor-sharp mind which soon afterward mercilessly recorded all confessions in official State Department memoranda.

He was named to this position on December 11, 1929, in large part because he had a private income sufficient to defray the costs of an ambassadorship while the State Department salaries and funds provided for entertainment were so low.

[8] He presented credentials on January 24, 1930, and left on May 27, 1930, but in that short time had developed a sympathetic view of Japan's foreign policy.

The day before departing he laid the cornerstone of a new American Embassy in Tokyo to replace the structure destroyed in the 1923 Great Kantō earthquake.

Naval Chief of Staff Admiral Kanji Katō refused to attend a farewell dinner for Castle.

Yeiji Kusakari, another Japanese Naval officer, committed the traditional suicide known as Seppuku, widely thought to be in protest of the treaty.

Their only child, Rosamond Castle, was born March 4, 1904, married Alan Francis Winslow on October 20, 1923, and died February 26, 1932, leaving three young sons.

[22] His house in Washington DC, designed in 1929 by Carrere & Hastings, survives on 2200 S Street NW in the Kalorama neighborhood.