Onassis was born in Smyrna, Ottoman Empire (now Izmir, Turkey) to the wealthy family of Socrates and Penelope Onassis. At the time of his birth, Smyrna had a significant Greek population. After being briefly occupied by Greece (1919 – 1922) in the aftermath of the allied victory in World War I, the city was recaptured by Turkey, and the Onassis family holdings were lost, spurring them to move to Greece as refugees. Plans for Aristos to head to Oxford were scrapped; instead, the young man caught a boat to Argentina, taking along the sum of 250 dollars. In his new adopted country, the emerging businessman's first investment move was in tobacco. By the time he purchased his first steamship in the early 1930s, Onassis was worth 1 million dollars. By 1932, Onassis had become a world-class businessman, owning commercial ships, tankers and whalers. He founded Olympic Airways (today Olympic Airlines), the Greek national carrier, in 1957 (he transferred the airline to the Greek State in 1975). In 1954, the FBI investigated Onassis for fraud against the U.S. government. He was charged with violating the citizenship provision of the shipping laws which requires that all ships displaying the American flag be owned by United States citizens. Onassis pled guilty and paid $7 million.
He married Athina Mary Livanos, daughter of shipping magnate Stavros Livanos, on December 28, 1946; their son, Alexander (April 30, 1948 – January 23, 1973), and daughter Christina Onassis, were born in New York City. After their divorce, Athina married her late sister's widower—and Onassis's arch shipping rival—Stavros Niarchos.
Despite the fact they were both married, Onassis and opera diva Maria Callas embarked on a notorious affair. According to Greek Fire: The Story of Maria Callas and Aristotle Onassis by Nicholas Gage, Callas gave birth to their child, a boy, who died hours later on March 30, 1960. Onassis ended his relationship with Callas to marry Jackie Kennedy, the widow of U.S. President John F. Kennedy, on October 20, 1968.
In 1963, he acquired the little barren island of Skorpios, which he later made his summer vacation.
Onassis died in Neuilly-sur-Seine, France, of bronchial pneumonia at the age of 75. Alexander had earlier died in an airplane crash. Christina inherited his fortune, which has since passed to her only child, Athina Roussel. He was buried in the island of Skorpios where Alexander had been buried two years before. Christina is buried in the same island.