In more than a half-century as a Teamster, William J. McCarthy learned the highways around New England and the paths to power and did it with a tough but low-key style.

"He's not the type to make a lot of publicity," said Robert DeRusha, secretary-treasurer of the Joint Council 10, which represents 55,000 Teamsters in New England. "He works hard and cares about the union movement. He's respected by everyone here, but he keeps a low profile."Such anonymity may be difficult now for McCarthy, 69, who was selected president of the nation's largest labor union Friday by the Teamsters' 17-member General Executive Board over Secretary-Treasurer Weldon Mathis, who Jackie Presser had named acting president May 4. Presser, 61, who died of cancer Saturday, had been under federal indictment on charges of labor racketeering and embezzlement.

McCarthy takes control of a 1.6 million-member union that routinely makes headlines for its influence in political campaigns and reputed links to organized crime.

McCarthy is one of 19 present and former Teamster leaders named defendants in a government suit trying to seize control of the union through a court-appointed trustee, claiming its leadership is either controlled by organized crime figures or incapable of removing its influence.

Among the allegations in the suit, filed two weeks ago, is that mob bosses in Chicago and New York were behind the elevation of Roy Williams to the union's presidency in 1981 and supported Presser in 1983.

McCarthy, who has a reputation of being a tough negotiator with a quick temper, will carve out the remainder of Presser's five-year term, which expires in 1991, when the union holds its next international convention.

Apart from the latest federal takeover suit and several investigations of alleged Teamster misconduct in the early 1980s, McCarthy has ruled the New England Teamsters without much public scrutiny.

McCarthy began his full-time union activity in the early 1940s negotiating contracts and handling grievances as the youngest business agent ever at Local 25 in Boston's Charlestown section.

By the mid-1960s, he had risen to secretary-treasurer of the New England Joint Council and was elected president four years later. He will retain the regional post in addition to his national responsibilities, said DeRusha.

Upward career mobility also allowed McCarthy to move from Charlestown to suburban Arlington, where he raised four children and lives with his wife, Mary.

In early 1982, McCarthy was called to testify in a 31-count federal case against three members of Teamsters Local 25 accused of defrauding movie outfits filming in Boston by arranging pay for absent union members.

The next year, McCarthy and eight other trustees in the New England Teamster Pension fund agreed to pay $205,000 as settlement of a U.S. Labor Department suit that alleged the trustees violated federal regulations by allowing the fund's investment adviser to receive commissions on real estate investments.

There are no other current Labor Department investigations that include McCarthy, according to a department spokesman in Boston.