Sirima Ratwatte was born on 17 April 1916 at the Ellawala Walawwa in Ratnapura. Her father was Barnes Ratwatte, later Dissawa of Sabaragamuwa, member of the State Council and the Senate of Ceylon; was a member of the Ratwatte family, which was an old Radala family hailing from the Kingdom of Kandy. Her mother was Rosalind Mahawelatenne Kumarihamy, daughter of Mahawalatenne Rate Mahatmaya of Balangoda. Her parental ancestors were courtiers in the courts of Sinhalese monarchs, and one, Ratwatte, Dissawa of Matale was a signatory of to the Kandyan Convention. Sirima was the eldest in a family six children, which included four brothers, Barnes Ratwatte II, Dr Seevali Ratwatte, Dr Mackie Ratwatte and Clifford Ratwatte, and one sister Patsy Ratwatte who married Colonel Edward James Divitotawela. The family resided at her maternal grandfather's Mahawalatenne Walawwa and later at their own Balangoda Walawwa. She remained a practicing Buddhist through her life.

Sirima Ratwatte first attended a private school in Balangoda, then moved briefly in 1923 to the primary school of Ferguson High School, Ratnapura, and was then sent to boarding school in Colombo at St Bridget's Convent. After completing schooling, she became involved in social work, distributing food and Medicine during the malaria epidermic of 1934/35, organizing clinics and helping empower rural industry.

In 1940, Sirima Ratwatte married Solomon West Ridgeway Dias Bandaranaike, a Oxford educated Lawyer turned Politician, who was at the time Minister of Local Administration in the State Council of Ceylon. Their wedding was dubbed "the wedding of the century", but caused a sensation within the Ratwatte family, as Bandaranaike was not considered to be an "acceptable" family. Sirima's marriage, however, was noted to have helped break down social barriers in Sri Lanka over the years. The newly married couple moved into Wentworth in Guidford Cresent, Colombo which was taken on rent from Lionel Wendt. There two children were born and the family stayed at Wentworth till 1946, when Sir Solomon bought a mansion at Rosmead Place, Colombo and the family settled into it.

Sirima was noted to have persuaded her husband to resign from government, resign from the United National Party, and establish the Sri Lanka Freedom Party in 1951. She campaigned for her husband during the 1952 parliamentary election, as her husband campaigned across the country. While the SLFP only won 9 seats during that election, Bandaranaike won his seat in Parliament.

During the campaign in the July 1960 elections, Bandaranaike toured the country and made emotional speeches, frequently bursting into tears as she pledged herself to continue her late husband's policies. That earned her the title "The Weeping Widow".

Bandaranaike was a socialist who continued her husband's policies of nationalizing key sectors of the economy, such as banking and insurance, and also nationalizing all schools then owned by the Roman Catholic Church in 1961. Bandaranaike was on a roller-coaster ride from the moment she took office. Within a year of her 1960 election victory she declared a "state of emergency". This followed a civil disobedience campaign by part of the country's minority Tamil population who were outraged by her decision to drop English as an official language and her order to conduct all government Business in Sinhala, the language of the majority Sinhalese. This they considered a highly discriminatory act and an attempt to deny Tamils access to all official posts and the law. This led to an increase in Tamil militancy which escalated under succeeding administrations.

Further problems arose with the state takeover of foreign businesses, particularly petroleum companies, which upset the United States and Britain; they ended aid to Sri Lanka. As a result, Bandaranaike moved her country closer to China and the Soviet Union and championed a policy of nonalignment. At home, she crushed an attempted military coup by Christian officers in 1962 and appointed her uncle william Gopallawa Governor-General of Ceylon. In 1964, she entered into a historic coalition with the Marxist Lanka Sama Samaja Party (LSSP). At the end of 1964, she lost a vote of confidence. The SLFP coalition was defeated in the 1965 elections, ending her first term as Prime Minister.

With the lapse of the 1964 agreement India declined to consider any more applications for citizenship. The government of Sri Lanka believes that the 1964 pact remains in force until the citizenship cases and permanent residence of all Indian Tamils covered by the pact have been settled. The All-Party Conference has agreed that the government should grant Sri Lankan citizenship to stateless Indian Tamils who did not apply for Indian citizenship and were not granted Sri Lankan citizenship under the 1964 agreement.

She regained power after the United Front coalition (SLFP, LSSP, and the Communists) won the 1970 elections with a large majority. But after just 16 months in power, the government was almost toppled by the 1971 JVP Insurrection of left-wing youths. Bandaranaike had disbanded the government's intelligence Service, suspecting that it was loyal to the opposition United National Party (UNP). Thus there was no warning of the uprising, and Sri Lanka's small army was caught off guard. The army mobilized its reserves and held the capital, but some outlying areas were occupied by the insurgents. The government was saved by military aid from both India and Pakistan, thanks to Bandaranaike's skillful foreign policy. In those tough political years, she turned herself into a formidable leader. "She was the only man in her cabinet", one of her officials commented during the height of the insurgency.

Her second term saw a new constitution introduced in 1972, replacing the Soulbury constitution. Ceylon was renamed Sri Lanka and became a republic. The 1973 oil crisis had a traumatic effect on the Sri Lankan economy: the government had no access to Western aid and her socialist policies stifled economic activity. Rationing had to be imposed. Bandaranaike became more and more intolerant of criticism and forced the shut-down of the Independent newspaper group, whose publications were her fiercest critics. Earlier she had nationalized the country's largest newspaper, Lake House, which has remained the government's official mouthpiece. Bandaranaike enjoyed continued success in foreign affairs. She was chosen chairwoman of the Non-Aligned Movement in 1976, and hosted the Movement's conference, attended by numerous heads of state. Despite her high international standing, she was losing Sri Lankan support rapidly amid allegations of corruption and against the background of a rapidly declining economy.

Under the Soulbury constitution, election would have been held in 1975. However, the government had become very unpopular. Bandaranaike used a clause of the 1972 constitution to delay elections until 1977. This did not help and in the 1977 elections the United Front was routed, winning only 8 seats.

Bandaranaike held her own seat in 1977, but in 1980, she was charged with abuse of power, expelled from parliament, and banned from office for seven years. Her rights were restored in 1986, and she ran for the post of President in 1988, but was narrowly defeated.

Under the terms of the 1964 agreement with India, 600,000 Indian Tamils were to be repatriated, while 375,000 were to be granted Sri Lankan citizenship. By 31 October 1981, when the two countries were to have settled this issue, India had taken more than 300,000 persons as repatriates. Sri Lanka had granted citizenship to over 185,000 plus over 62,000 post-1964 offspring. Over 207,000 Indian Tamils in Sri Lanka before 1964, plus nearly 45,000 offspring, were granted Indian citizenship but still awaited repatriation. In the wake of the July 1983 anti-Tamil violence, some in this latter group are being processed for repatriation or have emigrated to India.

In 1994, the SLFP-led coalition won the general elections. Chandrika then outmanoeuvred her mother to become prime minister; and then was elected President the same year. Bandaranaike became prime minister again, but constitutional changes in the years since her last term meant the office has been transformed into a mostly ceremonial post. Despite the office's lack of powers, Bandaranaike was noted to have wielded great influence among her party's faithful supporters. She remained in office until August 2000.

It was noted by the BBC in 2000 that Bandaranaike and Chandrika did not get along, largely due to leadership issues. Chandrika was known to be keen on Bandaranaike leaving, in order to make way for a younger face.