The former chief financial officer for Youth Pride Inc. yesterday pleaded guilty in U.S. District Court to three tax violations and revealed he has agreed to cooperate fully as a prosecution witness in the government's long-standing investigation of the self-help organization and its business enterprises.

As part of a plea-bargain agreement, government prosecutors have said they will not bring any charges against the financial officer, Johnny Mickens III, related to his dealings with Youth Pride or any of its spinoff organizations, including P.I. Properties Inc. The government reserved the right, however, to prosecute Mickens for perjury or any other charges that could be brought against him if he lies during his testimony or fails to cooperate with their investigation.

Mickens, 35, who now lives in Hampton, Va., had been director of administration for Youth Pride and a director and officer of P.I. Properties. He had close control over finances for both organizations, including check-writing authority and supervision of financial records.

The charges against him are the first criminal violations brought as a result of the government's long and complex investigation of Youth Pride, which shut down its business last August after 14 years of operation.

The now two-year-old federal investigation has centered on allegations that between 1974 and 1978 Youth Pride's former executive director, Mary Treadwell, and two other officers of P.I. Properties, Robert E. Lee Jr. and Joan M. Booth, stole, diverted or misappropriated $600,000 from the federal government and low-income tenants at Clifton Terrace Apartments, which was operated then by P.I. Properties.

Prosecutors in the case have refused to discuss whether or not indictments are expected as a result of the investigation.

As part of his written agreement with the prosecutors, which was filed in court yesterday, Mickens has agreed to testify before the grand jury and in any trials or other proceedings involving Youth Pride.

Mickens pleaded guilty yesterday to misdemeanor charges that he failed to file income tax returns and pay more than $16,000 in taxes owed for 1977, 1978 and 1979 on a total income of more than $70,000. He faces up to one year in jail and a $10,000 fine on each of the three charges.

Assistant U.S. Attorney William D. Pease said in court yesterday that in 1976, Mickens filed a "W-4" form with the Internal Revenue Service in which he claimed he was entitled to 99 tax exemptions. Mickens' high exemption claim on that form, which employers use to gauge the amount of salary to be withheld and paid to the federal government to offset upcoming tax bills, guaranteed that no money would be withheld from his paychecks.

Pease, who has been supervising the Pride investigation with assistant U.S. Attorney Stephen R. Spivack, said in court that Mickens, who has a degree in accounting, prepared tax returns for a number of Pride officials, including 1974 and 1975 tax returns for Treadwell, who was his close associate.

Mickens also prepared a corporate tax return for P.I. Properties, Pease said, and prepared tax returns for other people during the same years that he failed to file returns himself, Pease said.

Pease said Mickens was single, "living well" and spending large amounts of money during the years he failed to file tax returns. Pease told Judge John Garrett Penn during the hearing yesterday that in 1977 Mickens purchased a $6,000 boat but later told IRS agents that he had been unable to pay his federal taxes.

Mickens' lawyer, Barry Leibowitz, disputed Pease's characterization of the failure to pay taxes in 1977 and said Mickens was short of money because of complications with a real-estate transaction involving Treadwell and Lee and a subsequent civil lawsuit involving that transaction.

Mickens refused to comment yesterday about his decision to plead guilty in the tax case. Leibowitz told reporters after the hearing that Mickens has not yet paid any of the taxes he owed from 1977 to 1979 but intends to pay all of the amount. Leibowitz said Mickens currently has a house for sale in the District and "as soon as it is sold, it the tax will all be repaid."

Penn scheduled sentencing in the case for Feb. 22, but agreed to prosecutors' request for a possible delay if Mickens has not yet completed his cooperation with the government or testimony by that date. According to court records, the 18-month working term of the federal grand jury that has been hearing evidence in the case expires on Feb. 25.

Mickens was released on his personal recognizance pending sentencing.