The victory of Mahathir, 92, is considered a political earthquake that toppled the country's scandal-plagued premier and ousted a regime that had ruled for over six decades.
While the Thai government has apparently not expressed its felicitations to Mahathir, former PM Thaksin, who is wanted on criminal charges in Thailand, was quick to express his best wishes in a series of @ThaksinLive tweets.
"My sincere congratulations to Dr Mahathir Mohamed on his spectacular comeback. The people have spoken loud and clear that they not only remember his outstanding legacy but need his leadership," he wrote.
He misspelled Dr Mahathir's family name, spelling it Mohamed instead of Mohamad. The mistake appeared in his statements on his Facebook wall "Thaksin Shinawatra".
"From my experience of working closely with him, I am confident that he will lead the Alliance of Hope and Malaysia into a successful future," he wrote.
"His leadership will undoubtedly reinvigorate Asean as well as the ideas and visions he helped shape. I am certain that all the Thai people share my sentiment and respect towards him."
Thaksin also posted a photo of him shaking hands with Mahathir during an Apec Summit in Bangkok in 2003. His statements in twitter and Facebook were in both Thai and English.