Contemporary newspaper articles reporting on the 1917 appraisal of his father's estate state that William received a Trust Fund of $4,612,086 ($109,789,986.59 in 2023 when adjusted for inflation[1]), a life interest in $400,000, as well as the Congressional Gold Medal presented by the 38th US Congress to William's great-great grandfather Cornelius 'the Commodore' Vanderbilt I after the US Civil War which had come to signify the Headship of the Vanderbilt Dynasty.

During his service in the Navy, Vanderbilt served on the torpedo test ship USS Vesuvius from April 17 to May 31, 1917, the Naval Torpedo Station in Newport from June 1, 1917 to March 7, 1918, aide for information Second Naval District from March 7 to July 15, 1918, in Norfolk, Virginia from July 23 to September 16, 1918, New London, Connecticut from September 19 to November 14, 1918 and as a plank owner of the newly commissioned destroyer USS Evans from November 11, 1918 to August 30, 1919.

When he turned 21, the then legal age of majority, in 1922, Vanderbilt inherited a $5 million trust fund plus the 450 acre (1.8 km2) Oakland Farm in Portsmouth, Rhode Island, one of his father's estates that included a number of thoroughbred horses.

In 1925, Vanderbilt started a coach bus company, called The Short Line, carrying passengers between Newport and Providence.

The Short Line's original terminal building in Newport still stands and is located near the intersection of Spring and Touro streets.

His refusal to dole out patronage to fellow Republicans, however, weakened his power base, and a scandal over wire-tapping by a private detective firm he had hired to investigate election fraud, cost him re-election in 1940.

In May 1941 Vanderbilt, an officer in the Naval Reserve, was called to active duty in June 1941 with the rank of lieutenant commander and initially assigned to the Panama Canal Zone.

In May 1944 he was assigned to the staff of Admiral Chester W. Nimitz, commander in chief of the Pacific Fleet, in Pearl Harbor, Hawaii.

[9] After his discharge from the Navy at the end of the war, Vanderbilt left Rhode Island and retired to a farm in South Williamstown, Massachusetts.

Oakland Farm and its 150 acres in Portsmouth, Rhode Island was sold to Robert R. Young and divided into housing lots by the end of the 1940s.