The body of the Rwanda-based engineer was found in Kieni Forest in Gatundu North some 20km from his home about two weeks ago.
Police say the suspect's clothes and mobile phone were found in the victim's home in Mang'u, Kiambu.
The suspect is said to have worked for Mwangi for more than five years and on the Saturday night, he disappeared.
Charged with murder alongside the businessman's widow Gladys Njeri, alias Chania, in a Kiambu court is another farmhand, Morris Mbugua.
They were jointly charged with murdering the late contractor on October 9, in Mang'u, Gatundu North.
A postmortem revealed that Mwangi was hit several times at the back of his head.
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