In 1903, after completing medical studies, he was appointed Acting Assistant Surgeon, USN. In 1912, he was was assigned to the Bureau of Medicine and Surgery with additional duty as Aide to the White House.
Commissioned rear admiral in 1916, he served as medical officer for Presidents Theodore Roosevelt, William Howard Taft and, during World War I, Woodrow Wilson, for which he was awarded the Navy Cross. He was also made Commander of the National Order of the Legion of Honor by the French government.
Always interested in horses, he bought Blue Ridge Farm in Middleburg, Virginia’s oldest thoroughbred breeding farm, and lived there the rest of his life; his descendants still live there today. Also, the Grayson Foundation—predecessor of today’s Grayson-Jockey Club Research Foundation, Inc. was named in his honor as one of the first to support equine research.
After retiring in 1928, RAdm. Grayson was twice (1933, 1937) inaugural committee chairman for President Franklin Delano Roosevelt and served as head of the League of Red Cross Societies from 1935 until his death due to illness at Washington, 15 February 1938.