Fontenay was enrolled in the
Royal Danish Navy as a volunteer cadet at the age of five, as was custom at the time amongst the naval nobility. He was made a
cadet in 1735,
second lieutenant in 1741,
first lieutenant in 1746,
captain lieutenant in 1753,
captain in 1755,
commander-captain in 1763,
commander in 1769, and
counter-admiral in 1770. From 1744 to 1747, he was in the service of the
English Navy, participating in the
War of the Austrian Succession. From 1751 to 1754 he was on the
frigate Bornholm as second-in-command on tour in the
Danish East Indies. In 1753, the
Bornholm's Captain Sievers died, and Fontenay assumed command. After returning to Denmark, he became the commander of the frigate
Christiansborg. In 1755 he travelled to the
Danish West Indies, on a mission to escort 150 people, mostly families, across the
Atlantic. He returned to Copenhagen the next year. In 1771, he took over for command at
Holmen Naval Base, after Captain Frederik Reiersen was suspended for causing unrest there. He was sent to
Kristiansand in 1774 to investigate the feasibility of constructing a naval base there. On 3 June 1778, he was naturalized as a member of the
Danish nobility. In 1781, he was appointed as a deputy to the Admiralty Board, and following administrative changes, was then made deputy of the Admiralty and Commissariat Board in 1784. In 1782, he was decorated as a White Knight (Danish:
Hvid Ridder) within the
Order of the Dannebrog. == Personal life ==