Onassis died in hospital as a result of injuries sustained in an air crash at Hellinikon International Airport at the age of 24.

They had hoped that he might remarry their mother, which had seemed possible towards the end of their father's relationship with the Greek opera singer Maria Callas.

He also disapproved of Alexander's secret relationship with Fiona von Thyssen (née Campbell Walter), a British fashion model some 16 years his senior and the former wife of industrialist Baron Hans Heinrich Thyssen-Bornemisza.

[15] Alexander died on January 23, 1973, at the age of 24, from injuries sustained the previous day when his personal Piaggio P.136L-2 amphibious airplane, in which he was a passenger, crashed at Hellinikon International Airport in Athens.

[16] Alexander was instructing a potential new pilot of the plane, Donald McCusker, at the time of the crash, in his role as President of Olympic Aviation.

[16] Aristotle Onassis considered having his son's body cryogenically frozen with the Life Extension Society,[20] but was persuaded against it, and he was embalmed by Desmond Henley.

[22] Reports into the crash by the Greek Air Force and an independent investigator hired by Onassis, the Englishman Alan Hunter, concluded that it had occurred as a result of the reversing of the aileron connecting cables during the installation of a new control column.

This conclusion was disputed by McGregor who believed that the wake turbulence from an Air France Boeing 727 that had taken off before them had caused the crash.

[23] Less than a month after Alexander's death, McCusker had manslaughter proceedings initiated against him by the public prosecutor of Athens in connection with the crash.

[24] Six people were also charged over Onassis's death in January 1974, with their indictment indicating that faulty controls had been fitted to his plane.

[25] In December 1974, in a paid advertisement, Aristotle Onassis announced his offering of a $1,000,000 reward (equivalent to $6.178 million in 2023) for proof that his son's death had been as a result of "deliberate action" as opposed to the cause of negligence, the conclusion reached by the official inquiry.

[26][27] All charges relating to the crash were later dropped, and McCusker was awarded $800,000 in 1978 by Olympic Airways, three years after Aristotle Onassis's death.