A federal judge yesterday appointed attorney John Nields Jr. of the Washington law firm of Howrey & Simon to represent Mary Treadwell, the former executive director of Youth Pride, Inc., who was indicted on fraud charges more than five months ago.

Nields took over Treadwell's case yesterday after two other law firms, Covington & Burling, and Wilmer, Cutler & Pickering stepped aside because of potential conflicts between the Pride case and other clients.

Nields was the special government prosecutor in 1980 in the case of two former high ranking FBI officials who were convicted of conspiring to violate the civil rights of friends and relatives of members of the radical Weather Underground in the early 1970s by authorizing secret break-ins at their homes.

At a hearing yesterday, U. S. District Judge John Garrett Penn also appointed former government prosecutor Michael J. Madigan to represent Robert E. Lee, a former official of PI Properties, a real estate spinoff of Youth Pride.

Lee, Treadwell and two other former PI Property officials, Joan M. Booth, who is Treadwell's sister, and Charles W. Rinker Jr., were indicted on charges that they used PI Properties to defraud the government and low-income tenants of thousands of dollars which they used for personal expenses and to finance private, profit-making ventures.

Penn had agreed to appoint defense lawyers paid by the court for Treadwell and Lee after they said they were unable to pay attorneys to defend them against the government's allegations. All four defendants have denied any wrongdoing.