He was appointed a midshipman from the Georgia's 1st congressional district on 20 June 1919, attended the United States Naval Academy at Annapolis, Maryland, and graduated in the class of 1923.

Hobby reported to the Federal Shipbuilding and Dry Dock Company at Kearny, New Jersey, on 11 May 1939, to supervise the fitting out of the new Sims-class destroyer USS Anderson and to become its first commanding officer when it was placed in commission.

After Oklahoma capsized and sank in the Japanese attack on Pearl Harbor on 7 December 1941, Hobby served briefly in the 12th Naval District before he joined the new battleship USS Washington on 3 January 1942.

He acted as navigator of Washington until he relieved Commander Walter E. Moore as executive officer of the light cruiser USS Juneau at Espiritu Santo on 2 November 1942.

[3] After conversion to her new role, she was simultaneously christened—sponsored by Miss Catherine Hobby, sister of the ship's namesake, Commander William M. Hobby—and commissioned on 4 April 1945.

Shifting briefly to San Pedro, California, William M. Hobby returned to Oceanside and disembarked Underwater Demolition Team 29 on 13 August 1945.

Reaching Wakayama, Japan, on 28 September 1945, William M. Hobby soon got underway for Hiro Wan, Honshu, with Underwater Demolition Team 5 embarked.

That morning, William M. Hobby sighted a floating naval mine off her port bow and destroyed it with 40-millimeter, 20-millimeter, and rifle fire.

Proceeding via Guam, Eniwetok, Pearl Harbor, San Diego, and the Panama Canal, she arrived at Philadelphia, Pennsylvania, on 9 December 1945.