Official results showed that Mahathir's Pakatan Harapan (Alliance of Hope) had won 113 of parliament's 222 seats, clinching the simple majority required to rule. Najib's ruling coalition, Barisan Nasional (BN), had 79. Mahathir told a news conference he expected to be sworn in as Prime Minister later on Thursday (May 10).
Mahathir Mohamad will become the world's oldest elected leader, after a shock victory in Malaysia's election. The former Prime Minister, 92, came out of retirement and switched to the opposition to challenge his former protege Najib Razak, who has been beset by corruption allegations.
His historic win has ousted the Barisan Nasional (BN) coalition, which has been in power since independence in 1957.
Mr Mahathir said his coalition would "restore the rule of law". Mr Najib has not yet commented on his dramatic loss.
Official results showed the opposition Pakatan Harapan - the Alliance of Hope - had secured 113 of the 222 being contested, including some which have never before been taken by the opposition. BN took 79 seats.
- Source: News agencies