John Njiru Njue was found guilty of defiling the two minors at Kithimu village on June 23, 2015.
He was convicted for intentionally and unlawfully defiling the girls- aged 5 and 7 years.
Njue was also booked for indecently touching the girls' genitalia at his farm house.
The prosecution produced seven witnesses who proved beyond doubt that he committed the offence.
The girls, who are the old man's neighbours, were under the care of their aunt the day they were defiled.
They went out to play and strayed into the old man's farm before the latter lured them to enter his house.
The court heard that the man, who was with other workers, left the farm only to return after three hours.
Witnesses told the court that the girls were writhing in pain, and upon inquiry, told their mother of the incident.
They then led their mother and other neighbours to Njue's home after which police arrested him and was charged.
Senior resident magistrate Vincent Nyakundi said the prosecution witnesses corroborated their evidence.
Nyakundi dismissed Njue's defence that the girls were framing him, saying the young one had nothing to gain from doing so.
He will serve 40 years for the two offences consecutively.
-The star