Accountant Ronald S. Williams was fined $5,000 and placed on probation for three years yesterday for giving false information to federal prosecutors investigating allegations of fraud involving a real estate affiliate of the now-defunct Youth Pride Inc.

Williams told prosecutors last year he was not involved in a 1976 audit of the financial affairs of the spinoff firm, P.I. Properties Inc., when actually he had tried to audit the company's books and found they were in too much of a disarray to do so.

The 38-year-old Williams, now a senior corporate auditor for CBS in New York, pleaded guilty in April to charges of giving false statements.

Williams is the divorced husband of Mary Treadwell, the former head of Youth Pride and P.I. Properties who faces a trial next year on charges that she stole or misappropriated thousands of dollars from federally funded low-income housing projects she helped manage.

As part of his plea-bargain arrangement with the government, Williams has agreed to testify for the prosecution against his ex-wife and two other former P.I. Properties officials, Robert E. Lee and Charles W. Rinker Jr.

Williams, a tall, bearded man who was dressed yesterday in a dark green, three-piece pinstripe suit, cried as he hugged his attorney, Paul B. Bergman of New York, his stepfather, Robert Wilson, and sister, Dorisa, after being placed on probation.

The accountant had faced up to five years in prison and a $10,000 fine and Assistant U.S. Attorney Stephen R. Spivack had asked U.S. District Judge John Garrett Penn to impose "some period of incarceration."

"I'm pleased. Just leave me alone," Williams said as he left the courtroom.

Bergman told Penn that in retrospect Williams should have told Treadwell earlier than he did, in mid-1977, that P.I. Properties' financial ledgers were not auditable because of the poor condition in which they were kept.

But as an accountant, Williams "did exactly what he should have," Bergman argued. "He informed the principals" that their books could not be audited.

Spivack, in asking that Williams be sent to jail, contended that Williams "allowed himself to be used to cover up the deception, misappropriation and fraud" of others.

The prosecutor said that when investigators interviewed Williams "we expected him to tell the truth and he did not."

Williams was the second person connected with P.I. Properties to be sentenced for criminal offenses growing out of a nearly three-year-old federal investigation. Earlier this week, Penn placed Johnny Mickens III, a former director of the organization, on three years' probation for failing to file income-tax returns and pay more than $16,000 in taxes he owed from 1977 to 1979.

No trial date has been set for Treadwell, Lee and Rinker.