Official results showed that Mahathir's Pakatan Harapan (Alliance of Hope) had won 118 of parliament's 222 seats, exceeding the simple majority required to rule.
Meanwhile, the ruling coalition Barisan Nasional (BN) headed by Prime Minister Najib Razak won only 79 seats.
The Election Committee has yet to announce the official results of the election.
Mahathir declared victory at a meeting in Petaling Jaya city and said he plans to be sworn in as the country’s Government leader on May 10.
With this result, Mahathir will become the oldest elected leader at 92 years old in the world.
Mahathir told the media that the coalition had been contacted by the Palace, which said it would recognise the results pending an official announcement from the Election Commission (EC).
He also announced May 10 and 11 will be declared public holidays.
Mahathir also disclosed that he plans to appoint Wan Azizah Wan Ismail, President of the People’s Justice Party (Parti Keadilan Rakyat -PKR), as Deputy Prime Minister, making her the first female Deputy PM of Malaysia.
Mahathir served as Malaysian Prime Minister from 1981 to 2003.