He was administered oath by his younger brother and President Gotabaya Rajapaksa at a ceremony held at Kelaniya Mahaviharaya near Colombo.
Other cabinet members will be sworn in next week at a ceremony in Kandy. Mahinda Rajapaksa has served as country’s President from 2006 to 2015 and his party has got landslide victory at general elections held on Wednesday.
Rajapaksa is credited for ending the decades long LTTE war in 2009 during his Presidency.
He had lost the 2015 Presidential elections and has made a strong political comeback. Mr. Rajapaksa was appointed prime minister after Gotabaya’s victory at last November Presidential elections though his side did not enjoy majority in parliament.
He contested the election from Kurunegala and topped the preferential votes from the district by clinching record 5,27,364 votes.