Sultan Mohammad administered the oath of oath to him at Raj Palace in the capital Kuala Lumpur just before 10pm local time.
His oath ceremony from the Palace was broadcast live on TV. Mahathir was dressed as the country's traditional dress 'Baju Malaiu' and 'Surung' and the cap was on the cap.
Mahathirai is the world's oldest leader elected to the world.
Mahathir, who took the road to progress to Malaysia, fought against his former team in this year's general election. The leader of the opposition party, who was in power for 22 years, as leader of the opposition alliance, pulled down the party's Prime Minister Najib Razak from power.
Through this, Barisan Nasional (BN) Alliance's 61-year rule in Malaysia has ended. Since independence from Britain in 1957, the coalition was in power in the country continuously.
Mahathir will now get elected from the Opposition coalition and will get the Malaysian reelection.
On the occasion of his oath, Hundreds of people stood in line with the palace towards the palace, and the party flagged the hand.
The Election Commission declared Mahathir's victory on Wednesday night, but doubts arose during his oath taking ceremony.
However, Mahathir said on Thursday that a sworn affidavit will be held on Thursday. He said, "There is a matter of hurry. We need to form a government nowadays. "
Rajaprasada, in a statement, today dismissed Mahathir's appointment as the prime minister of the Malaysian constitutional monarchy.
It has been said in the statement that "the great King strongly supports and promotes the democratic process of the country and the will of the people."
Mahathir ruled the country from 1981 to 2003. After coming to power, his favorite candidate Najib Razak said.
But after nearly a decade of rajak got involved in corruption, Mahathir returned to politics to return to power and to return to power, he had to get back to the country.