Hipkins built a reputation for competence in tackling COVID-19, though he acknowledges some mistakes and faces a tough battle to retain power in an October general election.
The Labour party elected Hipkins following the surprise resignation of Jacinda Ardern last week.
The announcement triggered jubilation in the commercial capital Colombo where protesters massed outside the presidential secretariat, defying a citywide curfew.
An immigration official said Rajapaksa, his wife and two bodyguards left the country by a Sri Lankan air force plane in the early hours of the day he was due to resign.
A rundown of the major steps that led demonstrators storming the president's residence and setting fire to the prime minister's home in Colombo.
Two brothers who had dominated Sri Lanka's politics for nearly 20 years have gone their separate ways.
He replaces the president's brother, Mahinda Rajapaksa, who resigned amid deadly street protests.
Sri Lanka has been suffering its worst economic crisis in history, with a severe shortage of foreign exchange stalling essential imports, including drugs and fuel.
Hindus and Muslims have argued for decades for control over a religious site Mathura, echoing other quarrels between the two groups in India that have flared into deadly riots.
Foreign countries are greeting the makeup of the new government in Afghanistan with caution and dismay.
