Habsburg service
In 1505, at 23 years of age, Krsto Frankopan entered the military service of Maximilian I of Habsburg, Archduke of Austria and King of the Romans. He fought in the War of the League of Cambrai (1508–16) against the Republic of Venice and he won many battles including taking Devin and Pazin (1509). In battle, he distionguished himself as a soldier and leader, and for this, in 1510, Maximilian I, now Emperor of the Holy Roman Empire, gave him Podgrad and some possessions of the city of Postojna, and appointed Krsto an imperial adviser. In 1511, he was wounded in the face during the attack on the castle of Muggia near Trieste.
In 1512 and 1513, with his father, Krsto tried to take back the ancestral island of Krk from the Venetians who took it from the Frankopan family in 1480. In the second half of 1513 and the first half of 1514, he once again went into battle against the Venetians and took a large area of Friuli, including Monfalcone, Cividale del Friuli, Udine but the Venetians later took them back. In June of 1514, during an attack on Marano (Gradiška) he was captured and imprisoned in the “La Torresella” in the Doge's Palace. The imprisonment was not in a dungeon but in comfortable quarters and he had to pay his own expenses. Krsto expects to be released for a ransom, but due to his nigh status, he is kept captive by the Venetians until 1519, when the Venetians agree to hand him over to Francois I, King of France, who wanted to exchange him for a relative being held in captivity by Charles I, King of Spain. Krsto was moved to Milan, where on October 14, 1519, he bribed two guards and escaped.
Soon after his escape, Krsto returned to Habsburg service. After the death of Emperor Maximilian I (January 12, 1519), in 1520, he traveled to Emperor Charles V's court to confirm the estates given to him previously, which Charles does. Krsto was also appointed Captain of Rašpor (military commander of Istra) and the Karst (Kras) region. Frankopan then lived in Maran (Gradiška) and in the royal court. After Archduke Ferdinand did not fulfill some of his promises, on January 15, 1523, with Ferdinand's permission, he returned to his family estates in Croatia. Together with his father, he tried in vain to recover the family estates (Senj and others) that were taken from their family by King Matthias in 1469. Krsto entered the service of King Lajos II. In September of 1523, in the place of his sick father, Krsto went to Rome to ask Pope VI Hadrian to help Croatia fight the Turks. He gave a speech titled "Oratio Ad Adrianum Sextum...".
In June 1525, on his own initiative, Krsto led a mission to deliver food to the besieged fortress at Jajce, for which King Lajos called him "Protector of the Kingdoms of Croatia, Dalmatia and Slavonia", and Europeans called him "Hero of Jajce". At the Diet of Hatvan, held on July 4, 1525, many nobles asked for Frankopan to be appointed Ban of Croatia but King Lajos (Louis) did not agree. Again, in vain, Frankopan tried to have the city of Senj returned to the Frankopan family. After an argument between Krsto and László Szalkai [hu], the archbishop and chancellor of Esztergom, and primate of Hungary, escalated into a fight during which the archbishop grabbed Krsto by his beard and in response, Krsto slapped, possibly punched, him in the face. For this Krsto was imprisoned for three days. Following his release, Krsto returned to the service of Archduke Ferdinand.