Dr Mahathir said he had handed in his resignation letter to Prime Minister Datuk Seri Najib Razak recently.
Citing age as his reason to resign, Dr Mahathir said it was time for him to leave the national oil corporation to concentrate on his health.
"I have been with Petronas for 10 years, I feel that Petronas has performed well and made huge profit.
"I hope Petronas will continue to play its role in nurturing young entrepreneurs," he told reporters after hosting a luncheon for Umno International Forum 2013 participants here today.
Dr Mahathir Mohamad was discharged from the National Heart Institute on November 25 after being admitted on November 16 for a chest infection.
When asked whether he will also resign as Proton Holdings Bhd adviser, Dr Mahathir said he was not thinking about it yet. — Bernama