Okello was a member of the Acholi people and was born in 1914 in Kitgum district. He served since 1940 in the King's African Rifles of the British Army and took part in battles in East Africa. He was later commander of military units, as the Ugandan rebels of the Uganda National Liberation Army Ugandan dictator Idi Amin overthrew together with the army of Tanzania on 11 April 1979.
He was the successor of Bazilio Olara Okello, the 1985 for two days was head of state after the overthrow of Milton Obote. In January 1986, he was overthrown by the National Resistance Army under the current president, Yoweri Museveni.
His son Henry Oryem Okello is currently Minister of Education and Sports.
Until 1993 he lived in exile until President Museveni granted him amnesty. Okello returned to Kampala. Three years later he died in 1996 age of 82 at a unspecified illness.
In January 2010, Okello was what honored posthumously that he helped to overthrow Idi Amin. He was the Kagera National Medal of Honor awarded