Malawi's President Peter Mutharika had ordered the arrest of Eric Aniva, a sex worker known locally as a "hyena", after he admitted in a BBC interview to having sex with more than 100 women and underage girls and not disclosing his HIV status.
His lawyer, Michael Goba Chipeta, said Aniva would appeal against the conviction and the sentence.
The practice of "widow cleansing", when a widow must have sex after her husband dies, was outlawed a few years ago.
Aniva was the subject of a BBC feature into various sexual cleansing practices in Malawi.
The president had wanted him tried for defiling young girls, but none came forward to testify against him.
Instead Aniva was tried for "harmful cultural practice" under section five of Malawi's Gender Equality Act for having sex with new widows.
In some remote southern regions of the country it is traditional for girls to be made to have sex with a man after their first menstruation.
Last year Malawi banned child marriage, raising the legal age of marriage from 15 to 18 - something activists hoped would put an end to early sexual initiations.