Juan de Omedes, of the language of Aragon, Bailiff. of Caspe, was elected, on the 20th October 1536, to succeed as Grand Master of Malta on the death of Didier de St. Jaille.
Several disastrous expeditions against the Turks took place during the reign of Omedes. The arms of the Order sustained heavy losses at Susa and Monastir, and an attack on Algiers was repulsed by Barbarossa, who at the time was King of Algiers and tributary prince of the Sultan of Turkey. The Christian army, however, was successful in rescuing Tunis from the hands of Barbarossa, and would no doubt also have succeeded in taking Algiers had the wise suggestions of Admiral Doria been acted upon. Inflated by the success of the Turkish arms, the King of Algiers besought the Sultan to proclaim war against the Knights of St. John, suggesting that the island of Malta and the neighbouring stronghold of Tripoli should be taken off their hands and added to the Turkish Empire. The position of Tripoli had undoubtedly been rendered very precarious by the utter defeat the arms of the Order had sustained in their expedition against Algiers. Negotiations were speedily entered into with the Sultan Soliman and a large fleet of galleys was accordingly placed under Dragut, Governor of St. Maura, and a strong army under the command of Sinam Pacba, with orders to proceed to Malta and Gozo. The allied forces of Barbarossa and the Sultan arrived in Malta on the 16th July 1551 and marched at once to attack the city of Notabile. Vargagnon, of the language of France, was dispatched with six Knights to assist the besieged. The presence of the well known Knight inspired with the greatest courage the beleaguered inhabitants. The acclamations and the cries of joy of the people at the sight of their favourite Knight, led the Turks to think that a strong reinforcement had arrived from the Bourg. At the same time the false report that a Christian fleet had arrived for the relief of Malta was spread about, and Sinam Pacha beat his retreat from the island and tried a descent on Gozo. Galatian de Sesse, of the Langue d’Auvergne, was at the time Governor of the island. Unable to sustain a siege, he capitulated, and, together with 6,000 inhabitants, was put in chains and embarked on the Turkish galleys, which soon set sail for the coasts of Barbary.
The eyes of Sinam were now turned to Tripoli, which he eventually won from the hands of the Knights who were under the command of Governor Gaspard de Vallier. Murat Aga was invested with the title of King of Tripoli and Tagiora, he giving the right of succession to Dragut.
On the loss of Tripoli the Grand Master at once took measures to ensure the safety of Malta. A motion having been brought forward before the Council on the 8th January 1552 by Leon Strozzi, Prior of Capua, to provide for the further defence of Malta, it was unanimously carried, and as a result Fort St. Michael was built and Fort St. Elmo reconstructed.
The Grand Master, far advanced in years, died on the 6th September 1553, after reigning seventeen years.