He was the son of Nichols Pratt, who served in the Union Navy during the American Civil War with the rank of acting master.

After graduating from the United States Naval Academy in 1889, Pratt served in several cruisers and gunboats, visiting Europe, South America and Asia.

Pratt's work with the U.S. Fleet was interrupted in early 1930 by a trip to England to participate in the London conference that further limited the size of the world's major navies.

He became Chief of Naval Operations in September 1930 and spent nearly three years in that post, during a time when Depression-era demands for economy made it very difficult to maintain the Navy's size and readiness.

During World War II, he wrote a regular column for a nationally circulated magazine and spent several months on active Navy Department duty in 1941 studying measures to counter the German submarine threat.

Like his father, Pratt was a companion of the District of Columbia Commandery of the Military Order of the Loyal Legion of the United States.