Stephen Tanabe, 50, of Alamo, was found guilty in federal court in San Francisco of one count of conspiracy, three counts of wire fraud and two counts of extortion.
He was acquitted of one charge of extortion.
The charges stem from his role in helping former private investigator Christopher Butler, 52, of Concord, set up the arrests of three men in stings in Danville between November 2010 and January 2011.
The men, who were husbands of Butler's female clients in divorce and child custody cases, were allegedly enticed by employees associated with Butler to become intoxicated at Danville bars and then arrested after Butler alerted Tanabe that the drunken men were driving away.
Tanabe stood trial for three such arrests.
The jury of four women and eight men found Tanabe guilty of extortion for receiving a Glock gun worth roughly $600 from Butler in exchange for making arrests.
Defense attorney Tim Pori said the jury acquitted Tanabe of the second extortion charge, which stemmed from allegations that Butler also gave him $200 worth of cocaine as payment for the "dirty DUIs."
The wire fraud charges stem from text messages he and Butler exchanged the evenings of the setups.
Butler and former Contra Costa County narcotics squad commander Norman Wielsch were the ringleaders of a larger corruption scheme that included stealing drugs seized as evidence and extorting protection payments from workers at a massage business they set up.
Last year, Butler was sentenced to eight years in prison after pleading guilty to seven charges including illegal wiretapping and conspiring to sell methamphetamine and marijuana Wielsch had stolen from evidence lockers. As part of his plea deal, he agreed to testify against Tanabe.
Wielsch pleaded guilty in 2012 to five charges, including conspiracy to distribute marijuana and methamphetamine, and was sentenced to 14 years in prison.
Tanabe is scheduled to be sentenced on Dec. 11. He faces a maximum of 20 years in prison and is out on bail, Pori said.