Callaghan was born on July 26, 1890, in San Francisco, California, the son of businessman Charles William Callaghan and Rose Wheeler Callaghan. The family was devoutly
Roman Catholic. One of his younger brothers,
William Callaghan (1897–1991), would later go on to a career in the US Navy as well. Both brothers studied at
Saint Ignatius College Preparatory in San Francisco, the elder graduating in the class of 1907. His first assignment was on board the
armored cruiser , in command of a
turret with twin guns. He was promoted to the rank of
ensign on May 21, 1912. His second assignment was on the
destroyer in mid-1913. He was promoted to
lieutenant (junior grade) in May 1915. During his first few years of naval service, Callaghan had been courting Mary Tormey of
Oakland, California; the two married on July 23, 1914. was born in
Alameda, California, on October 16, 1915. In July 1915,
Truxtun was on its way to Alaska when it broke down and could not continue its mission. Initially, the blame fell on Callaghan, who had apparently ordered incorrect condenser parts. He was suspended from duty and ordered to appear before a
court-martial. Subsequent investigation, however, found that another man was responsible for the error, and Callaghan received a full acquittal and was reinstated. A few months later, he was appointed as commanding officer of
Truxtun, but the stress of his trial appeared to have left its mark—at the age of 25 years, his hair had already turned gray. ==World War I==