From a noble family in Saint Malo, Magon became a garde marine in 1777. His father, the governor of the Mascarene Islands, died in 1778 and left his son his estate of Médine on Mauritius, from which Magon derived his full name. He fought at Ushant in 1778 on the ship of the line "Bretagne" before participating in the English Channel campaign on the "Saint Esprit". Rising to enseigne de vaisseau in 1780, he served in the Antilles, on the "Solitaire", in the comte de Guichen's squadron. He fought in three battles against Rodney's fleet off Dominica before serving in de Grasse's squadron on the "Caton", fighting at the Chesapeake, St. Kitts and Les Saintes. He was captured and only freed at the end of 1782.