The siblings are credited with destroying in brutal fashion ethnic Tamil separatists to end Sri Lanka’s civil war a decade ago when Mahinda was president and Gotabaya effectively ran the security forces.
Mahinda, the elder and more charismatic of the brothers, was unable to run for president, having already served the maximum of two terms between 2005 and 2015.
Thursday’s nationally televised brief ceremony — held deliberately at an astrologically auspicious time — came after Ranil Wickremesinghe stepped down in the wake of Saturday’s election.
Gotabaya, 70, was elected president on Saturday thanks to his popularity among the Sinhalese-Buddhist majority, thrashing his main moderate rival despite support from minority Tamils and Muslims.
He campaigned with promises to make the island of 21.6 million people safe in the wake of suicide bombings by homegrown Islamist extremists in April that killed 269 people.
Mahinda was also prime minister twice between 2004 and 2005 and again for 52 days during last year’s political crisis before the Supreme Court ruled his appointment unconstitutional.
Official sources said the 74-year-old Mahinda would name his cabinet on Friday. He will likely head a minority government and elections cannot be held before March.
Around 40,000 Tamil civilians were allegedly killed by the military in 2009 but the Rajapaksas have denied allegations of war crimes and resisted pressure for an international probe.
During Mahinda’s 2005-15 presidency, Sri Lanka also moved closer to China, borrowing almost $7 billion for infrastructure projects — many of which turned into white elephants mired in corruption.
Two other Rajapaksa brothers, Basil and Chamal, are also active in politics. Basil is also credited with leading Gotabaya’s election campaign while Chamal is a member of parliament and a former speaker.
© Agence France-Presse