I am sad and angry to see The Post perpetuating old myths and unjust stereotypes regarding mental illness.

In the story ''District Agency Hires Lawyer Suspended for Mental Disorder'' {Metro, Aug. 21}, The Post seems to presume that the hiring of attorney James E. Mercer, who, as reported, was diagnosed as suffering from a bipolar mood disorder, is newsworthy. In bold print, The Post quoted Maudine Cooper, director of the Office of Human Rights: ''He told me about his problems. . . . I didn't ask to see the psychiatric report.''

This story perpetuates a social stigma which presumes that once one is diagnosed with a mood disorder, one can never function as a normal human being again. The Post does great injustice to the thousands of people in every walk of life who are suffering or have suffered from emotional disorders.

According to the Center for Community Development of Prince George's County, 2.1 million Americans will be affected by a manic episode in their lifetime. Three or four times that number will have major depressive episodes. Only one in five people affected ever seek treatment. Almost 30 million Americans suffer daily from the pain of mental illness or emotional handicap. Yet, the federal research dollar per patient for fiscal year 1987 is $14 for schizophrenia and $10 for depression.

Why not rejoice that Mr. Mercer ''voluntarily committed himself to St. Elizabeths Hospital last September''? Why not rejoice that he was found qualified for his new position and is now functioning well? People do recover.

The Post's bias about mental illness comes through loud and clear. It does not help those of us who seek to treat and to support the mentally ill and their families. What is so terribly tragic is that The Post is missing a great opportunity to become part of a larger solution, while so many people base their impressions on the images it holds before us.

STEPHEN K. KUMMERNUSS

Pastor, Hope Lutheran Church

Clinton