Onn's father was Jaafar Muhammad, the former Chief Minister of Johor. His mother was Roquaiya Hanim (also spelled Rogayah Hanim or Rukiye Hanım; 1864–1904), who came from the Caucasus region of the Ottoman Empire (there are different hypotheses regarding her ethnicity). She was likely presented as a concubine (see Circassian beauties) by the Ottoman court to the sultan of Johor. His mother was married three times and the last time was with his father. As Onn Jaafar's family had close relations with the Johor palace, Sultan Ibrahim treated him as an adopted son. He started his education in a Malay school in Johor Bahru. In 1904, he went to England to attend Aldeburgh Lodge School, a private school in Suffolk, with the then Tengku Mahkota until 1909. He excelled in sports and captained the school’s cricket and football teams.