The first mental patient to win a constitutional right to liberty will get damages of $20,000 from two state psychiatrists who refused to free him for 14 1/2 years even though they knew he was harmless and got no treatment.

The damage award may lead lawyers to seek the release of thousands of other mental patients who were committed against their will and who are dangerous neither to themselves nor others.

That prospect - not the cash - accounts for "my happiness today," said 68-year-old Kenneth Donaldson, who had been kept in a chairless, crowded, ward in Florida State Hospital in Chattahoochee from 1957 to 1971.

He and his lawyers met with reporters yesterday less than two hours after federal Judge William Stafford in Tallahassee approved a settlement with the widow of former hospital superintendent J.H. O'Connor and with John Gumanis, the staff physician who "treated" him.

In addition to the $10,000 each defendant must pay Donaldson within 60 days, Judge Stafford may order them later to pay attorney's fees - estimated at between 150,000 and $200,000 - under a 1976 federal law.

In behalf of the Mental Health Law Project, a non-profit, Washington-based group, and the American Civil Liberties Union, Bruce J. Ennis Jr. represented Donaldson for six years, and Paul R. Friedman, managing attorney of the law project, reprsented him for three years.

They told a press conference in the law project's offices that an award of attorney's fees would help to make it possible for others to try to free involuntarily committed patients.

They cited data indicating that about 500,000 such persons are committed annually, some for brief periods. About as many are in institutions at any one time.

Overall, they said, 90 to 95 per cent of such patients - a higher proportion than in the population at large - are not dangerous. The key problem is that in identifying the patients it's unsafe to free, psychiatrists admittedly have a batting average of only about 5 per cent.

In an unaninous decision on the Donaldson case in June 1975, the Supreme Court held that a mere judgement of mental illness "cannot justify a state's locking a person up against his will and keeping him indefinitely in simple custodial confinement" if he "is capable of surviving by himself" or with the help of family or friends.

Three years earlier, a jury had awarded Donaldson $38,500, including $10,000 in punitive damages. Acting on a challenge by the physicians, the Supreme Court, while proclaiming the "right to liberty" for patients such as Donaldson, sent the case back to the trial court on the issue of damages. The settlement was reached two weeks before the scheduled start of a second trial.

Donaldson, who lives in York, Pa., wrote a book last year "Insanity Inside Out," Crown, lectures, and is working on two more books, one of them an account of his experiences in fiction form. "I'm retired," he said with the sense of humor that pervaded his exchanges with reporters.

His parents had committed him. The initial diagnosis was that he was schrzophrenic/paranoid, had delusions, and was possibly dangerous.

He slept - when "screaming and hollering" inmates permitted - in a 60-bed room. For a year, he worked in a locked kitchen from 6 a.m. to 7 p.m., seven days a week. He got no pay. In 14 1/2 years, doctors spoke with him for a total of not more than five hours.

Like most mental patients, he was too poor to hire a lawyer. He learned to do his own legal work, but his requests for a hearing were rejected 18 times by state and federal judges and three times by the Supreme Court.

He asked the help of 50 separate lawyers in "sensible, coherent" letters, Ennis said. Understandingly, he said, they turned him down because none would or could afford to spend thousands of dollars to try to win the first money damages from a doctor for violation of a constitutional right.

When Donaldson filed his third appeal in the Supreme Court, physician-lawyer Morton Birnbaum represented him without fee. Then the ACLU entered the case with a friend-of-the-court brief.