Mesih is first attested in historical accounts in 1470, by which time he was the "sanjak bey" of Gallipoli. As the chief Ottoman naval base, the post also entailed command over much of the Ottoman fleet. In this capacity he distinguished himself in the Ottoman conquest of Euboea (medieval Negroponte) from the Venetians during the Ottoman–Venetian War of 1463–1479. However, Venetian archives document that soon after he offered to surrender Gallipoli and its fleet to Venice, in exchange for 40,000 gold ducats and the possibility to become ruler over the Morea (the Peloponnese peninsula in southern Greece), which had long been a semi-autonomous despotate under the Palaiologoi emperors.