John W. Nields Jr., the tall, serious-minded court-appointed defense attorney for Mary Treadwell, is no stranger to major legal battles, but in the past he has been a government prosecutor seeking convictions, not acquittals.

Nields, 40, served as chief counsel to the House ethics committee during the panel's investigation of the so-called Koreagate scandal, involving allegations of influence buying on Capitol Hill by South Korean Tongsun Park. In the end, a handful of congressmen were disciplined for accepting cash payments from Park and one went to jail.

Nields' work in developing evidence in the case did not go unnoticed. Although he had joined the private law firm of Howrey & Simon after leaving the ethics committee position, former attorney general Benjamin R. Civiletti appointed Nields as the special prosecutor in the government's controversial conspiracy case against two former high-ranking FBI men, W. Mark Felt and Edward S. Miller.

Nields won convictions against both men for authorizing illegal searches--"black bag jobs"--at the homes of friends and relatives of radical Weather Underground fugitives in the early 1970s. President Reagan pardoned both men shortly after he assumed office, an action Nields sharply criticized at the time.

In another recent case, Nields was one of the lawyers who successfully argued against the Reagan administration's plan to require parents to be notified when their daughters received birth control devices.

Nields, a graduate of Yale and the University of Pennsylvania law school and once a law clerk for Supreme Court Justice Byron White, easily transferred his prosecutorial skills--and at least one personal habit--to the defense table in the Treadwell case.

He proved a tenacious courtroom battler, wasting few opportunities to poke holes in the weak points of prosecution witnesses.