The activities were part of a scheme to steer at least $15.8 million worth of UAW merchandise contracts to vendors, including Ashton's personal chiropractor, according to federal court documents.
Two other former union officials who participated in the conspiracy, Jeff Pietrzyk and Mike Grimes, already pled guilty to charges of money laundering and are thought to have provided information to federal prosecutors in exchange for leniency at sentencing.
Ashton was the union's representative on the GM board of directors for three years. He abruptly resigned from the board in August 2017 as the federal investigation in corruption in the UAW picked up momentum.
The UAW, which is midway through contract negotiations with Detroit's "Big Three" automakers, denounced the alleged conduct.
"Joe Ashton's actions, as set forth in the government's filings, are completely inexcusable and violate UAW's long-standing standards of conduct put in place by former leaders like Walter Reuther," UAW Acting President Rory Gamble said in a statement.
"The UAW remains focused on negotiating and finalizing strong contracts for our members, especially during this round of auto negotiations."
About a dozen current and former UAW officials and auto industry executives have been charged in the probe, which centers on revelations that they enriched themselves through the Center for Human Resources, a joint body run by the auto companies and the union that was meant to train workers.
UAW President Gary Jones took a leave of absence from the union on Saturday, without explaining his departure. The FBI raided Jones' home in August as part of the investigation. Last month the union negotiated a new contract with General Motors, after the longest automotive strike in the US in nearly 50 years. And last week, the UAW said it reached a tentative labor agreement with Ford, and it plans contract talks with Fiat-Chrysler.