Additional district judge Nikesh Kumar Sinha held Singh, husband of Dhanbad Municipal Corporation chairman Indu Singh, guilty under section 302 of IPC and Section 27 of Arms Act in the murder of trade union leader Binod Singh on July 15 in 1998.
The quantum of punishment would be announced on April 16.
Singh was not present in the court at the time of hearing due to health reasons.
Binod Singh was gunned down in day light while he was on coming here from Katras.
The court exonerated Bachha Singh from the charges. Rajiv Ranjan Singh has not appeared in court since 2004 and has been declared absconder. Sher Bahadur Singh was also declared absconder in the case.