Onassis was born in Smyrna (now Izmir), a city in what is now Turkey. At the time of his birth, Smyrna and had significant amounts of Greek population and culture. After being briefly occupied by Greece (from 1919 to 1922), the city was recaptured by Turkey, and the Onassis family holdings were lost, spurring the family to move to Greece as refugees. By 1932, Onassis had become a world-class businessman, owning commercial ships, tankers, and whalers. He founded Olympic Airways, the Greek national carrier, in 1957.
He married Athina Mary Livanos, daughter of shipping magnate Stavros Livanos, in 1946; they had a son and daughter, and were divorced in 1960; she then married the Marquess of Blandford, heir to the dukedom of Marlborough, and later still, her late sister's former husband, Stavros Niarchos. For a number of years, Onassis's mistress was the opera singer Maria Callas. He married, as his second wife, Jackie Kennedy, the widow of U.S. President John F. Kennedy, on October 22, 1968.
Onassis died in Paris of the effects of myasthenia gravis in 1975. His son Alexander predeceased him; his daughter Christina's only child, Athina Roussel, was the sole inheritor of the bulk of his fortune.
His name in Greek is written as Αριστοτέλης Ονάσσης.