His lasting legacy began in April 2006 when he sided with Maoist fighters and led mass street demonstrations that forced the former king, Gyanendra, to relinquish dictatorial powers, reinstate parliament and appoint Koirala as caretaker prime minister.
Koirala's government stripped Gyanendra of all his powers and command of the army.
The peace deal signed with the former rebel Maoists in 2006 ended a conflict that had killed at least 16,000 people, with thousands more still unaccounted for.
Soon after that Koirala stepped down as prime minister leading to the Maoists winning landmark elections in 2008, abolishing the 240-year-old Hindu monarchy and transforming the impoverished country into a secular republic, bringing democracy to Nepal.
Koirala had been dogged by ill health in recent years but he remained heavily involved in politics until his death, playing a crucial role as broker between the Maoists and other parties as the peace process faltered.
Koirala often remarked that the process was "the last struggle of my life" and joked that he would come back as a ghost to haunt Nepal if it failed.
He led the Himalayan nation through some of its biggest upheavals including its most notorious upset when 10 members of the royal family were shot dead by the crown prince in a drunken rampage.
He was seen as a stabilising force in a country that has seen 18 governments in the last 20 years, although like many politicians in Nepal he faced frequent allegations of corruption.
Koirala's body will lie in state at the national stadium from Sunday, with his funeral to be held later in the day at the Pashupatinath Hindu temple in Kathmandu, his aide said.