He inherited the title Duke of Aosta on 3 March 1942 following the death of his brother Prince Amedeo in a British prisoner of war camp in Nairobi.

From 18 May 1941 to 31 July 1943, Aimone was designated king of the Independent State of Croatia (Croatian: Nezavisna Država Hrvatska, NDH), even though he never ruled there.

[2][3] Later, however, he refused to assume the kingship in protest of the Italian annexation of the Dalmatia region,[4] and is therefore referred to in some sources as king designate.

With his brother Amedeo, he was educated at St David's College, Reigate, Surrey, England, and Aimone later went to study at the naval academy in Livorno.

Due to the failure to climb K2 twenty years earlier, Prince Aimone's expedition concentrated solely on scientific work.

On 18 May 1941, in a ceremony at the Quirinal Palace, to which Ante Pavelić, the leader of the fascistic Ustaše movement that had assumed power in Croatia in April 1941 after the invasion of Yugoslavia, led a delegation of Croats requesting that Italy's King Victor Emmanuel III name a member of the House of Savoy as king of Croatia.

Prince Aimone also established a Croatian office in Rome where he received confidential reports, official documents, and military, political and economic information from Croatia.

[27] After the fall of the Fascist regime in Italy, Aimone abdicated as king of Croatia on 31 July 1943 on the orders of Victor Emmanuel III.