However, in 1985, at the age of 20, he left Makerere to join the National Resistance Army (NRA), in their guerrilla war against the Obote II regime (1980–1985) and the military junta (1985–1986) that ousted him.
[9] During the same period, the Ugandan security organs were accused of the creation and maintenance of "safe houses", where arrested suspects were detained incommunicado beyond the 48-hour limit prescribed by law, without any charges being brought in court.
[10] It was under Brigadier Mayombo's reign as Director of Military Intelligence, that the People's Redemption Army under Colonel Kiiza Besigye was first brought to the attention of Ugandans.
Brigadier Mayombo himself confronted Besigye over a radio talk show on KFM's Hot Seat programme one evening.
His condition deteriorated further and on Sunday 29 April 2007, comatose, on life-support systems, he was flown to Agha Khan Hospital in Nairobi, Kenya.
The Ugandan president, Yoweri Museveni, gave permission for the presidential Gulfstream IV-SP to be used to fly Mayombo from Entebbe to Nairobi.
[13] On Tuesday, 1 May 2007 at 15.00 local time, Mayombo died at Agha Khan Hospital in Nairobi of complications arising from acute pancreatitis.
[16] A three-person team was named to carry out the investigation; its members were Dr Peter Mugenyi, then the director of the Joint Clinical Research Centre, Colonel James Mugira, commander of the Uganda People's Defence Force Tank Unit, and Lieutenant Tagaswire Rusoke, a biochemist in the UPDF.
Following a period of public viewing at the Parliament Building in Uganda's capital, Kampala,[3] Mayombo's body was taken to Kololo Airstrip for a state funeral attended by Ugandan Cabinet members, senior members of the Ugandan military, diplomats accredited to Uganda and delegations from several countries including Rwanda, Kenya, Southern Sudan, the Democratic Republic of the Congo, the East African Community and South Africa.