Kathryn P. Casey, 73, a retired attorne;y with the Federal Trade Commission and a longtime resident of Washington died Tuesday at the New Rochelle Hospital Medical Center in New Rochelle, N.Y., after a heart attack.

Miss Casey, who was born in New York City, grew up in Washington and was a graduate of the old Western High School. She lived for a time in California before returning to the area in the mid-1930s as an administrative assistant for the old Coal Commission, now a part of the Department of Interior. During the time whe worked for the Coal Commission, she wrote a handbook on bituminous coal laws.

About 1941, she earned a law degree from the Columbus School of Law, now part of Catholic University. From 1942 until 1952, she was the general counsel and the Washington repreentative of the International Apple Associaiton, a trade organization. In 1947, while representing the IAA, she become the first woman to argue a case before the Interstate Commerce Commission.

Miss Casey joined the legal staff of the House Committee to Investigate Tax Exempt Foundations in 1952 and remained there for two years. In 1955, she became a staff lawyer with the Federal Trade Commission, working in the office of the assistant general counsel in charge of compliance. She remained there until she retired in 1972.

Miss Casey, who maintained residences in Washington and New Rochelle, was a member of the American Bar Associatio;n, the D.C. Bar Association, the American Judicature Society, the American Trade Association Executives, The Business and Professional Women's Club and the Daughters of the American Revolution. She was a member of the parish of St. Thomas Apostle Catholic Church in Washington.

Survivors include a sister, Mrs. Joseph W. Barker of New Rochelle.