Mathis's Teamster career began when he was elected business agent for Local 728 in Atlanta, Georgia, in 1950.
The United States Department of Labor was investigating him at the time due to allegations of vote fraud in his last re-election bid.
Mathis was considered much more liberal Presser or the rest of the Teamsters' leadership at the time, and he was much more in favor of participating fully in AFL-CIO.
Mathis was challenged by a faction of conservative Teamsters led by Joseph Trerotola, the union's First Vice President.
Trerotola was deeply angered by the 1986 constitutional amendment which allowed Mathis to assume the presidency, and he began building a coalition to oust him.
McCarthy—who led the opposition to the freight and warehouse contract—promised to fight the government's trusteeship suit, adopt a more confrontational collective bargaining posture, end the union's thaw toward the rest of the AFL-CIO, and endorse a George H. W. Bush in the 1988 presidential election.
In late December 1988, the federal government sued to overturn Mathis's 1987 election as president of Local 728.
He consented to aggressively seek internal reforms in the union in exchange for being dropped from the government's labor racketeering suit.
The Teamsters reached an agreement with the Justice Department on March 12, 1989, in which the union agreed to institute internal reforms in order to end corruption and improve the democratic nature of its elections.
In February 1991, Mathis backed a decision by Durham to reject a McCarthy-backed candidate for an empty vice presidential seat.
The move was seen as an attempt to put political distance between the Durham candidacy and the McCarthy administration, which was increasingly unpopular with Teamster members.
A week later, on February 8, 1991, Mathis recommended that the Teamster executive council look into the bidding process McCarthy used to award his son-in-law a contract to print the union magazine.
A court-appointed administrator of the Teamsters had ruled that McCarthy had improperly awarded the printing contract but did not seek charges against him.