Mexis was born circa 1756 in Spetses, the son of Theodoros Mexis who hailed from Lambovo, Northern Epirus. He amassed a large fortune as a ship owner and a merchant and gained much influence in Spetses. During the War of Independence, he used his fortune to finance the war effort, providing war ships and lending money to the revolutionary government.
He attended the First National Assembly as a representative of Spetses between December 1821 and January 1822. On September 8, 1822, he successfully led the effort to defend the island against an attempted Turkish invasion.