The practice of "widow cleansing", when a widow must have sex after her husband dies, was outlawed a few years ago.
Eric Aniva, a sex worker known in Malawi as a "hyena", admitted in a BBC interview to having sex with more than 100 women and underage girls and not disclosing his HIV status.
This led to the president ordering his arrest in July.
President Peter Mutharika had wanted Aniva 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.
Two women testified against him.
He will be sentenced on 22 November 2016.
The BBC's southern Africa correspondent Karen Allen says the case has attracted international media attention and sharply divided opinion as to how widespread the practice remains.
Aniva was the subject of a BBC feature into various sexual cleansing practices in Malawi.
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.
An HIV-positive Malawian man has been found guilty for having unprotected sex with newly bereaved widows.