Mary K. Rosinski, 77, a partner in the realty firm of Carl G. Rosinski Co., died Friday in Suburban Hospital in Bethesda after a brief illness.

Mrs. Rosinski worked until she was 74, retiring three years ago from the firm.

A native of Washington who attended the Academy of Notre Dame in Baltimore, Mrs. Rosinski became a licensed real estate broker in Washington, Maryland and Virginia after the death of her husband, Carl G. Rosinski, in 1946.

Her husband had founded the real estate firm in 1929. Mrs. Rosinski specialized in the leasing and management of business property after she joined the firm.

A member of the Washington Board of Realtors, Mrs. Rosinski was named an honorary lifetime member of the board in 1976.

She was a member of The Shrine of the Most Blessed Sacrament Parish in Chevy Chase for over 50 years. She also had belonged to the Ave Maria Unit of the Sodality since 1926, and was a member of the Christ Child Society.

Mrs. Rosinski is survived by a son, Donald C., of Bethesda; two daughters, Anne E. Redman of Chevy Chase, and Joan M. Eccard of Wheaton; a brother, William A. Mahon, of California, 12 grandchildren and five great-grandchildren.