Mr. and Mrs. Robert Harold Mitchell, Jr. of Old Greenwich, Ct. announce the engagement of their daughter, Katherine Fontaine, to G. Cabell Williams III, son of Mr. and Mrs. George Cabell Williams of Chevy Chase, Md.

Miss Mitchell is a graduate of the Greenwich Academy and of Rollins College, Winter Park, Fla. She is an account coordinator for J. Walter Thompson. Her father is engaged in private investment banking and counseling in Greenwich and New York and was formerly president of Matthews, Mitchell & Co., Inc. and a partner in Abbott, Proctor & Paine. She is the granddaughter of Mrs. James Pinckney Harrison and the late Mr. Harrison of Richmond and Charles City County, Va. Mr. Harrison was Chairman, Board of Directors of Universal Leaf Tobacco Company.

Mr. Williams, a graduate of Mercersburg Academy (Pa.) and Rollins College, is the Washington marketing representative for Wang Laboratories. He is the grandson of Mrs. Bolitha J. Laws of Washington and the late Chief Judge Laws and of the late Mr. and Mrs. George Cabell Williams of Danville, Va. The future groom's father is president of Allied Capital Corporation here and in Ft. Lauderdale, Fla. and is also on boards of Pandick Press, New York City and American Federal Savings and Loan Corporation in this city.

The wedding is planned in Old Greenwich, Ct. in February.