He takes over from Nderitu Gachagua, 64, who died early on Friday while undergoing treatment in Britain.
Gachagua had been battling pancreatic cancer, an illness that saw him admitted in the UK for two months in 2015.
Wamathai was expected to be sworn in on Saturday but no arrangements had been made from the Attorney General’s office by close of business on Friday.
Nyeri County Assembly Speaker Mwangi Mugo on Friday told the assembly that he was consulting with the AG’s office to ensure that the ceremony was conducted immediately to ensure there is no power vacuum.
Gachagua was the first governor to die in office. The Governor had been seeking re-election under the Jubilee Party.
Party officials said he had paid the nomination fee ahead of primaries in April.
President Uhuru Kenyatta described the late Gachagua as a great leader and an action-oriented and diligent man.
“His tenure boasts many accomplishments. In terms of infrastructural development, he oversaw the construction of over 1,000 km of gravel in Nyeri, which saw many areas, previously inaccessible during the rainy season, opened up,” said Uhuru.
“Under his tenure, citizens of Nyeri were finally able to conduct business late into the night — up to 10 pm — thanks to his efforts to improve street lighting in trading centers. Gachagua also registered progress in the area of education: initiating a bursary fund that now benefits 10,000 secondary students a year.”
Opposition leader Raila Odinga said in his statement that Gachagua remained committed to his people until death.
“I have very fond memories of the late Governor, having worked with him in and out of government in the past. I knew him as a smart politician with the humility, strength and determination to serve his people,” he said.