The former child soldier, now in his early 40s, goes on trial before the International Criminal Court (ICC) on Tuesday for crimes committed in Uganda, including keeping sex slaves and recruiting child soldiers.
Ongwen, known as the "White Ant", is the first leader of the brutal Ugandan rebel army led by the fugitive Joseph Kony to appear before the ICC, created to try the world's worst crimes.
The son of school teachers, he was abducted as a child before being forced into the rebel army and allegedly becoming a willing perpetrator of violence.
He rose swiftly through the LRA ranks, quickly being singled out for his murderous loyalty and tactical ability and taking command of one of the army's four brigades.
Ongwen is accused of carrying out massacres, rapes, mutilations and abductions in quick and lethal raids.
Ongwen's men -- with trademark dreadlocks, mismatched uniforms and AK-47 rifles fitted with bayonets -- also allegedly carried out thousands of abductions of children.
Boys were taken to be soldiers or porters, girls were taken as sex slaves or drafted into military ranks as fighters.
The LRA fighters were also notorious for punishment raids where they would slice the lips and ears off victims as a grim calling card.
Under the leadership of self-proclaimed prophet Kony, the LRA is accused of kidnapping tens of thousands of children during its nearly three-decade long insurgency.
Prosecutors told a January hearing in the ICC that Ongwen was the "tip of the spear" of the group that has sown terror across several countries in central and eastern Africa.
Between 2002 and 2003, Ongwen is thought to have directed bloody campaigns in northern Uganda that butchered or abducted thousands.
He is also accused of playing a central role in revenge attacks on civilians in the troubled Democratic Republic of Congo.
In recent years, however, he was reportedly sidelined after falling out with Kony over his execution of another commander.
Hailing from the northern Ugandan district of Gulu, Ongwen was known "as much for his volatile nature as his bravery", according to the LRA Crisis Tracker, which monitors the rebels.
Years of psychological trauma are also said to have taken their toll, with Ongwen earning a reputation for flying into murderous rages.
Wanted by the ICC for almost a decade, Ongwen surrendered to US special forces in the Central African Republic in January 2015 after Washington offered a $5-million (4.4-million-euro) reward for his capture.
Mark Kersten, a London-based academic focusing on international justice, has described Ongwen as "both a victim and a perpetrator of international crimes" and said efforts to prosecute him could raise difficult questions.
"When is a victim a perpetrator and a perpetrator a victim? The line is much more murky than we tend to assume," he said.