Mr. and Mrs. Ralph W. Kittle of Washington, D.C. and Keswick, Va. have announced the engagement of their daughter, Cary Marian, to David Finley Williams, son of Mr. and Mrs. Richard P. Williams of Washington D.C. and Leesburg, Va. The wedding is planned for September 20 in Grace Episcopal Church in Cismont, Va.

The future bride is an associate with the New York City law firm of Shearman & Sterling, graduated from the Chapin School, N.Y., and received undergraduate and law degrees from the University of Virginia. She spent her junior college year at the University of Salzburg, Austria.Her father is a vice-president of International Paper Company and director of its Washington, D.C. office. He is also Chairman of the Board of Trustees of Mary Baldwin College and is a trustee of Lee's College in Kentucky.

Mr. Williams, an associate with the Washington, D.C. law firm of Covington & Burling, graduated from the Episcopal High School in Alexandria. After completion of a year's study at Repton School in Derby, England with the English Speaking Union, he obtained his undergraduate degree from Yale University and law degree from the University of Virginia. His father is a partner in the Washington, D.C. law firm of Purcell and Nelson.

The future bridegroom is a grandson of the late David E. Finley, former Director of the National Gallery of Art here, and Director of the National Trust for Historic Preservation; and of the late Richard P. Williams, Jr., former teacher and Headmaster of the Episcopal High School.