"I cannot break (the manifesto pledge) at the moment," he said in a video posted on his party's Facebook page on Friday (May 18).
"Unless of course there is a demand that I take up the education portfolio."
The new education minister is former university lecturer Dr Maszlee Malik from Dr Mahathir's Bersatu party, he added in a press release later on Friday.
The U-turn comes a day after Dr Mahathir announced that he would also be the education minister, saying he felt this was something he should give attention to as "many people are uneducated in this country".
While the news was welcomed by many, others pointed out that Pakatan Harapan's manifesto states that "the Prime Minister will not simultaneously hold other ministerial posts, especially the post of Minister of Finance".
There were reports that leadership had not recalled this themselves in deliberations, thinking that the prime minister was only barred from the finance minister position.
A veteran leader of Pakatan Harapan's Democratic Action Party Lim Kit Siang said he was "stumped" himself.
"Apart from PM cannot be FM, not aware that PH manifesto proposed PM cannot hold another portfolio," he tweeted on Thursday.
In a media release on Friday, Dr Mahathir also announced the names of several other ministers, including those who will helm the ministries of transport, health, agriculture, housing and human resource.
A foreign minister has not been appointed.
Altogether, the names for 15 positions - including the prime minister and deputy prime minister posts - were submitted to the king. The names have been approved and they will be sworn in on Monday at 11.30am, said the media release.