[1] Little is known of Okello's youth: he was born in Lango District in what was the Uganda Protectorate, and was baptized at age two, receiving the baptismal name of Gideon.
[7] The highly religious Okello was convinced he had been given orders in his dreams by God to break the powerful position of the Arabs and to found a revolutionary state on Zanzibar and Pemba.
On the night before the revolution, Okello gave his men the order to kill all Arabs between 18 and 25 years of age, to spare pregnant and elderly women, and not to rape virgins.
[7] On 12 January 1964, with popular support from the island's native African majority, Okello and his men fought their way to the capital of Zanzibar, Stone Town, where the Sultan lived.
[9][10] The coup led to the little-known bloodbath of between 2,000 and 4,000 ethnic Arabs, South Asians and Comorians,[11][12] whose families had been living in Zanzibar for centuries, between 18 and 20 January.
Afterward, Okello appeared to be too unstable to play any role in government of the new country and was quietly sidelined from the political scene by Karume, who allowed him to retain his title of Field Marshal.
[21] The People's Liberation Army (PLA) was formed by the government in April and completed the disarmament of Okello's remaining FMF troops.
[22] Karume's reason for doing so may have been to prevent the radicals in the Umma Party from taking over the country or to reduce the possibility of increasing communist influence in East Africa.
[citation needed] But Okello's status as a member of the Lango group, alongside his popularity and charisma may have played a factor in his disappearance.
The black slave played by Edward Roland in Werner Herzog's 1972 film Aguirre, the Wrath of God is named "Okello".
In his commentary to the DVD version of the film, Herzog also says that the character of Aguirre himself was partly modelled on John Okello, with whom the director had been in contact.