Thornsbury, 57, of Williamson, faces a maximum of 10 years in prison and a $250,000 fine when Johnston sentences him. He remains free on $10,000 bail.
Thornsbury, who had served as the county's only circuit judge since 1997, resigned as part of his plea deal with prosecutors.
According to the federal charge against Thornsbury, late Mingo County Sheriff Eugene Crum -- described in the charge as "a close associate and political ally" of Thornsbury -- bought several thousand dollars' worth of signs and other promotional campaign items on credit from George White's sign shop in Delbarton.
After Crum was elected sheriff, instead of paying his $3,000 bill, he allegedly sent an undercover police officer to buy oxycodone tablets from George White.
After White's arrest, federal investigators approached his lawyer, former Williamson Mayor Charles "Butch" West, and asked to talk to White about allegations that he provided drugs to Crum. White told FBI agents that, on "multiple occasions prior to his arrest, he unlawfully provided Crum with prescription narcotic pills at Crum's request," prosecutors said.
Federal prosecutors said Crum soon learned what White had told FBI agents and that a meeting was arranged with White's brother. Prosecutors said Mingo County Commissioner David Baisden, Prosecuting Attorney Michael Sparks and Crum devised a scheme to get White to switch attorneys, in exchange for a lighter sentence from Thornsbury.
Senior Status Circuit Judge John Cummings vacated White's conviction earlier this month.
If U.S. District Judge Thomas Johnston approves Thornsbury's plea agreement, prosecutors will dismiss charges that Thornsbury violated the constitutional rights of his former secretary's husband by trying to land him in jail on trumped-up charges.
In Thursday's motion, federal prosecutors also said they want Thornsbury available for meetings, stating that if he were in prison it would make setting up meetings with him more difficult.