Koroma grew in Freetown, the capital of Sierra Leone. However, he was born in Tombodu, Kono District, in eastern Sierra Leone. He was a northern Limba, the same ethnic group as former Presidents Siaka Stevens and Joseph Saidu Momoh, who was deposed by the army in 1992. He joined the Sierra Leonean army in 1985, and went to Sandhurst Military Academy in England to train as an officer in 1988. He returned to Sierra Leone the next year and was promoted to Section Commander, and soon thereafter to Company Commander. He continued to move up the ladder, and in 1994 he went to the Teshi Military College in Ghana to train in the subjects of Army Command and General Staff.
Koroma received military training in Nigeria and Britain. He commanded government forces who were fighting against the Revolutionary United Front of Foday Sankoh. He was arrested in August 1996 due to his involvement in a coup plot against the southern civilian officials who were in control of the country. President Ahmad Tejan Kabbah was to have been killed. Koroma was freed from prison during a successful military coup on May 25, 1997, when soldiers loyal to him brought him to power. He advocated making a peaceful settlement with Sankoh and allowing him to join the government, though this never happened.
After the coup in 1997, Koroma was named Head of State and Chairman of the AFRC. Koroma invited the leadership of the Revolutionary United Front (RUF) to join the AFRC, which they promptly did. In order to maintain order, Koroma suspended the constitution, banned demonstrations and abolished all political parties.
The AFRC coup was accompanied by an explosion of violence against civilians throughout the nation; the key strategic change was that the RUF had immediate access throughout the country, something they had failed to achieve through six years of military action. To justify the AFRC coup, Koroma cited corruption, the erosion of state sovereignty, over-dependence on foreign nations, and the failure of the Government of Sierra Leone to adequately address tensions between the SLA and government supported tribally-based militia movements, in particular the Kamajors. Koroma's story is very much that of the AFRC, and indeed the failings of the Abidjan Peace Accord struck between the Government of Sierra Leone and the RUF on 30 November 1996.
By 2 June 1997, the RUF/AFRC found itself at odds with Nigerian forces, which were deployed unilaterally at this stage under the Economic Community of West African States (ECOWAS) Ceasefire Monitoring Group (ECOMOG) and its mandate of [August 1997). The Nigerians were stationed in and around Freetown's Western Area, trading mortar fire along the main highway into Freetown and around the Freetown International Airport. Koroma immediately sought to ease the situation, seeking mediation, which resulted in the signing of a peace accord in late October 1997 in Conakry, Republic of Guinea. Almost immediately, violations of the peace accord were perpetrated by all sides to the complex conflict in Sierra Leone. By January 1998, ECOMOG forces were preparing to oust the RUF/AFRC from power. On 6 February 1998, ECOMOG forces invaded key locations in the Western Area, removing the RUF/AFRC entirely by 12 February 1998. On 1 March 1998, ECOMOG forces commenced operations in provincial Sierra Leone, removing the RUF/AFRC from every key town except Kailahun (in the far east of the country). By December 1998, RUF/AFRC forces had reversed this position, invading Freetown in January 1999. Failing to hold territory, the RUF/AFRC retreated into the Northern Province of Sierra Leone.
The leadership of the RUF led peace negotiations with the Government of Sierra Leone led to the signing of the Lomé Peace Accord on 7 July 1999. Koroma was cut out of both negotiations and the AFRC did not benefit from the substantive provisions of the agreement. Nevertheless, Koroma participated in the disarmament process, encouraging those SLA soldiers that had joined the AFRC to demobilize. By 2000, Koroma no longer held significant influence over the RUF leadership, as evidenced by the involvement of ex-AFRC members (from a splinter group called the West Side Boys) in defending towns in Port Loko District against a renewed RUF offensive in May 2000. In August 2000, Koroma officially disbanded the AFRC and sought to consolidate his political position through the formation of a political party.
In early 2002, the Government of Sierra Leone and the United Nations signed a bilateral treaty establishing the Special Court for Sierra Leone, mandated to try those who "bear the greatest responsibility" for crimes against humanity, war crimes and other serious violations of international humanitarian law. According to the indictment, the RUF and AFRC, under the orders of Koroma, led armed attacks in Sierra Leone wherein the primary targets included civilians, humanitarian aid workers, and UN peacekeeping forces. These attacks served the purpose of terrorizing the population as a form of punishment for not supporting rebel activities. These attacks included such crimes as looting, murder, physical violence (notably mutilations), employing child soldiers, sexual violence and rape, as well as kidnapping women and girls to be raped or turned into sex slaves. Men and boys were also abducted and forced to work or fight for the rebel groups.
On 7 March 2003, the Prosecutor of the Special Court issued his first indictments. For his role in the RUF/AFRC, Koroma was among them. Koroma fled Freetown in December, reportedly to Liberia. On 1 June 2003 he was officially declared dead under mysterious circumstances, claimed to be murdered. The Prosecutor has yet to withdraw the indictment against Koroma. An October 2006 newspaper headline in Freetown stated "Johnny Paul has 1,000 armed soldiers".
According to a report in September 2008, Koroma's remains were found buried in Foya, a village in Liberia's Lofa County; this was unconfirmed, however.