 Dechaineux was born in Launceston, Tasmania, to a Belgian-born father, Florent Dechaineux, and an Australian mother.He entered the Royal Australian Naval College, Jervis Bay at the age of 14, graduated three years later, and was promoted to Midshipman in 1920.In the first half of the 20th century, the RAN worked very closely with the British Royal Navy (RN), frequently exchanging personnel.