Dewitt Coffman was born in Mount Jackson, Virginia, on November 28, 1854. He was appointed to the U.S. Naval Academy, Annapolis, Maryland, by the Honorable John Thomas Harris, Representative from the Sixth Virginia District in 1872. Graduated on June 20, 1876, he served the two years at sea, as then required by law, before he was commissioned Ensign to date from July 10, 1879. He advanced periodically in grade to that of Rear Admiral to date from December 12, 1914, and between June 19, 1916 and August 1918 he served in the temporary rank of Vice Admiral. On November 28, 1918, having reached the statutory retirement age of sixty-four, he was transferred to the Retired List of the U.S. Navy. On June 21, 1930, he was commissioned a Vice Admiral on the Retired List in accordance with the Act of that date which provided that officers be advanced in rank on the Retired List to the highest grade held by them during World War I.
Following graduation from the Navy Academy in 1876 he joined the USS Pensacola, flagship of the Pacific Fleet. In October 1877 he transferred to the USS Portsmouth and cruised around the Horn in her before he was detached with orders to the USS Franklin, receiving ship at Philadelphia, Pennsylvania. He had duty in the Franklin from June to October 1879, after which he was assigned to the USS Constellation and USS Trenton, successively. He served in the latter, flagship European Station, until December 1881, when he reported aboard the USS Colorado, receiving ship at New York, New York.
Detached from the Colorado in May 1882 he had six months duty with the Fish Commission, and after awaiting orders reported in January 1883 aboard the training ship New Hampshire. He was again assigned to the receiving ship Franklin, from September to December 1883, after which she joined the USS Yantic. During 1885 and 1886 he had service with the Southeastern Alaskan Survey aboard the coast survey steamer Carlisle P. Patterson, serving as Executive Officer of the vessel in 1887. In December of that year he reported for duty in the Hydrographic Office, Navy Department, Washington, D.C.
He rejoined the Pensacola, flagship of Commander, South Atlantic and South Pacific Stations, successively, in July 1890 and in March 1892 transferred to the USS Adams, employed to the Seals Island patrol in the Bering Sea. In January 1893 he assumed command of the sealing schooner Mountain Chief captured in the Bering Sea. As prize master he took that vessel to Unalaska in the Aleutian Islands, where she was turned over to the authorities. Between January and July 1893 he was assigned to the USS Boston, and while aboard commanded a company of the landing party from the Boston when the Provisional Government took charge of the Sandwich Islands.
Following a tour of duty at the Naval Academy (September 1893 to July 1896) interspersed with service in the training ship Bancroft between June and August 1895, he had consecutive service in the USS Texas, USS Columbia and USS Terror until June 1898. He was aboard the latter during the Spanish American War. After a period of hospitalization he reported in October 1898 aboard the auxiliary cruiser Prairie (ex USS El Sol), and in January 1899 transferred to the USS Marblehead. His brief service in that vessel ended in April of that year when he reported aboard the USS Indiana in the Caribbean, for duty as Watch and Division Officer.
In July 1899 he received orders to proceed home and in September was again assigned to the Naval Academy. He remained there until May 1900, when he joined the USS Newport as Executive Officer. From August 1900 until June 1901 he had a third tour of duty at the Naval Academy, after which he had instruction at the War College. In May 1900 he reported as Executive Officer of the training ship Essex, and remained aboard until she was decommissioned.
From February 1904 to September of that year he had duty at the Navy Yard, Norfolk, Virginia, and thereafter until October 1905 served as Officer in Charge of the Navy Recruiting Station at Baltimore, Maryland. He then assumed command of the USS Boston, commanding her until ordered to return to the United States. He became Inspector of Ordnance in Charge of the Naval Magazine at Fort Mifflin, Pennsylvania, in July 1907 and while in this assignment had duty under instruction at the Naval War College, Newport, Rhode Island. In July 1908 he reported as Officer in Charge of the Naval Magazine, St. Julien’s Creek, Virginia.
Between May 1900 and March 1911 he commanded the USS New Jersey, and from June 1909 served as Commander of Artillery, Atlantic Fleet. He then became Commandant of the Navy Yard and Station at Boston, Massachusetts. Relieved in November 1914, he had instruction at the Naval War College, Newport, Rhode Island. Upon completion of the course in December of the same year, he assumed command of the Third Division, Atlantic Fleet, with flag in the USS New Jersey. In March 1916 he transferred in the same capacity to the Sixth Division, Atlantic Fleet, USS New York, flagship. He continued to serve in that assignment until May 1916 when he was designated Second in Command of the Atlantic Fleet, with additional duty as Commander Battleship Force Atlantic Fleet, and Commander Division Eight, Atlantic Fleet.
After the entry of the United States into World War I in April 1917, he reported in July of that year as Commander Battleship Force Two, with additional duty as Commander Squadron Four, and Division Eight, Atlantic Fleet. He was awarded the Distinguished Service Medal for “exceptionally meritorious service in a duty of great responsibility as Commander of Battleship Force, Two, Atlantic Fleet, and also for the efficient manner in which you administered and operated the Fleet in the temporary absence of the Commander in Chief during August, September and October 1917.”
In August 1918 he reported as Commandant Fifth Naval District, Norfolk, Virginia, and was so serving when transferred to the Retired List of the U.S. Navy on November 28, 1918. However he remained on active duty as Commandant Fifth Naval District, until March 1919 when he was assigned to the Board which recommended those deemed worthy of the Medals of Honor, Distinguished Service Medal and Navy Crosses. He was detached as Commandant Fifth Naval District, by remained on the Board of Awards. On October 31, 1919 he was relieved of all active duty. On January 5, 1920 he returned to active duty on the Board of Awards and was again relieved of active duty upon the dissolving of the Board on June 30, 1920.