Hart was a sailor in the United States Coast Guard.
He first enlisted on September 3, 1924, when he was 26 years old.
He was promoted to Boatswain's mate and was in command of CG-213 when it effected a daring rescue of a stranded tugboat in November 1926.
During this rescue Hart dived into dangerous seas to rescue a member of the tug's crew who had fallen overboard.
According to his Coast Guard biography: "Hart jumped overboard and affected the rescue at great personal risk, as the two vessels were not more than 8 feet apart in the raging seas." For this act of heroism Hart was awarded the Gold Lifesaving Medal.
In 1927 his heroism in fighting a gasoline fire earned him a commendation.
In 1933 to 1939 Hart served in the US Army Corps of Engineers, where he rose to Warrant officer.
He re-enlisted in the Coast Guard in 1939, and was quickly promoted to Chief Boatswain's Mate.
When the US entered World War II Hart was promoted to Lieutenant.