Malaysia opposition hopes to be sworn in today
In a political earthquake, Mahathir's opposition alliance ended the governing Barisan Nasional (BN)'s six decade hold on power.
The victory capped a dramatic political comeback for Mahathir, who previously ruled the country with an iron fist for 22 years.
He came out of retirement to take on the once Saudi-backed Prime Minister Najib Razak after the leader became embroiled in a massive corruption scandal.
Mahathir, at 92 years old, will be the oldest prime minister in the world when he takes power. Concerns have mounted in the delay of transfer of power Thursday morning after Mahathir was not inaugurated by the king at 9am.
He said there had been some delays due to confusion over certain parts in the constitution, however, reassured his supporters that this had now been cleared up.
Speaking to a press conference, Mahathir said: "We expect today for me to be sworn in as prime minister".
He added that "there is an urgency here. Currently there is no government in Malaysia".
His speech came following Najib's comments who did not give a clear concession, saying it was up to the king to decide on the prime minister as no single party had won a clear majority.
"I accept the verdict of the people and BN is committed to the principles of democracy."
Mahathir's victory spells big trouble for Najib - Mahathir has vowed to bring him to justice over allegations that billions of dollars were looted from sovereign wealth fund 1MDB, which the scandal-plagued leader set up and oversaw.
Najib denied any wrongdoing and said the money was a donation from the Saudi royal family.
To claim a simple majority in parliament, a party or coalition would require 112 seats.
Pact of Hope, the opposition alliance, along with a small ally on Borneo Island, won 121 seats. BN won just 79 - down from 133 previously.
Confusion arose after the official Election Commission listed the scores for the alliance's separate component parties when they released the results, not the overall score for the alliance as a whole.