Additional District and Sessions Judge-II Awadesh Kumar Dube passed the order convicting Swamy, the son of former RJD MP Umashankar Singh, after finding him guity of abduction and murder of Bharat Singh, a brother of Damodar Singh, who was the JD(U) candidate from Maharajganj Assembly seat in 2000.
The court will pronounce the quantum of sentence on August 13.
Bharat Singh was abducted on February 15, 2000 and his body was recovered two days later. Following this, the victim's kin had lodged an FIR against Swamy charging him with the murder.
Swamy had, meanwhile, unsuccessfully contested the bypoll in Maharajganj Lok Sabha seat on a Congress ticket in 2013 following death of Uma Shankar Singh, who was a sitting MP from the constituency.
He had later joined BJP and had unsuccessfully contested from Daraunda Assembly seat in 2015.