Sushil Koirala was born to Bodh Prasad Koirala and Kuminidi Koirala on 12 August 1939 in Biratnagar, second-largest city of Nepal. Koirala was unmarried and known to live a simple life. A member of the politically prominent Koirala family, he was the cousin of former prime ministers Matrika Prasad Koirala, Girija Prasad Koirala and Bishweshwar Prasad Koirala.

Koirala entered politics in 1954 inspired by the social-democratic ideals of the Nepali Congress. In 1958 he keenly participated in Bhadra Abagya Aandalon, (Civil Disobedience Movement) launched by the Nepali Congress. In 1959, he actively involved himself in the party’s objective of carrying out the democratic elections. The election saw Bishweshwar Prasad Koirala become the first elected prime minister of the country. However, King Mahendra planned and executed a coup in December 1960 and expelled Bishweshwar Prasad Koirala. This resulted in the exile of several members of the Nepali Congress to India, which included Sushil Koirala. He remained in political exile in India for 16 years following the royal takeover of 1960. Koirala also spent three years in Indian prisons for his involvement in a plane hijacking in 1973. While in exile, Koirala was the editor of Tarun, the official party publication. He has been a member of the Central Working Committee of the party since 1979 and was appointed General Secretary of the party in 1996 and Vice President in 1998.

He was elected to the Pratinidhi Sabha from the Banke-2 constituency in 1991 and 1999 on a Nepali Congress ticket.

In 2001, he lost the contest for the Nepali Congress Parliamentary Party leader to Sher Bahadur Deuba. Koirala was appointed acting President of the party in 2008 by President Girija Prasad Koirala. On 22 September 2010, the 12th general convention of the Nepali Congress elected him as party President.

A heavy smoker, Koirala was diagnosed with tongue cancer in 2006 and lung cancer in June 2014. He died on 10 February 2016 at 12:50 AM of pneumonia in Kathmandu, Nepal, at the age of 76. He used to be known as ‘Sushil daa’. It is said that Koirala had a formal education of I.Com from a college of India according to his sister-in-law although he always mentioned his qualification as informal education.

Koirala lost in the 2008 Constituent Assembly elections from Banke-3 coming in third behind the candidates of the Madeshi People’s Rights Forum and the CPN (Maoist)

The Nepali Congress emerged as the largest party in the 2013 Constituent Assembly elections under Koirala’s leadership. He was elected leader of the Nepali Congress Parliamentary party securing 105 out of 194 votes against former Prime Minister Sher Bahadur Deuba’s 89 votes and on 10 February 2014 he was nominated as prime minister. During his tenure as prime minister, his government was criticized for its slow aid response to the April 2015 Nepal earthquake. A historic agreement among four major political parties was also made that year, which paved the way for a new constitution in the country. Honoring a pledge to stand down as prime minister once the new constitution came into effect, Koirala resigned on 10 October 2015. He sought re-election but was defeated by Khadga Prasad Sharma Oli, leader of Congress’ former coalition partners CPN-UML.