Johnny Mickens III, former chief financial officer for Youth Pride Inc., was placed on probation for three years yesterday for failing to file federal income tax returns and pay more than $16,000 in taxes he owed from 1977 to 1979.

Mickens is the first former Youth Pride official to be sentenced as a result of a lengthy federal investigation into the financial affairs of the self-help organization and its spinoff affiliates.

While Mickens has been cooperating with government prosecutors, it was unclear yesterday whether he will be called as a witness against his former Youth Pride boss, Mary Treadwell, who faces trial on charges that she stole or misappropriated thousands of dollars from federally funded low-income housing projects she helped manage.

Both Barry L. Leibowitz, Mickens' attorney, and Assistant U.S. Attorney Stephen R. Spivack said they do not know whether Mickens, 35, will testify.

Mickens was director of administration for the now-defunct Youth Pride and a director and officer of one of its sister organizations, P.I. Properties. He had close control over finances for both firms.

Mickens told U.S. District Judge John Garrett Penn that he was sorry for his failure to file the income tax returns for the 1977-79 period, during which he had an income of more than $70,000.

In 1976, Mickens filed a W-4 form with the Internal Revenue Service in which he asserted that he was entitled to 99 tax exemptions. The high exemption claim on that form, which employers use to calculate the amount of salary to be withheld and paid to the federal government to offset upcoming tax bills, meant that no money would be withheld from his paycheck.

Leibowitz told the judge that Mickens had paid $300 of the $16,000 in back taxes. Under terms of the probation, Mickens is required to use proceeds from the anticipated sale of a District house he owns to pay more of the taxes due.

Mickens was not indicted as part of the alleged conspiracy centering on the financial dealings of P.I. Properties, but his income tax illegalities were discovered during the broader investigation into Treadwell's activities.

Ronald S. Williams, Treadwell's estranged husband and Youth Pride's accountant, has pleaded guilty to making false statements to federal investigators in the case and is scheduled to be sentenced by Penn on Wednesday.

Treadwell's sister, Joan M. Booth, has pleaded guilty to conspiracy to defraud the government and tax evasion and is scheduled to be sentenced Feb. 1.

A trial date has not been set for Treadwell and two other former P.I. Properties officials, Robert E. Lee and Charles W. Rinker Jr.