John C. Flack, 83, retired manager of the insurance department of the Randall H. Hagner and Co. real estate firm in Washington, died of pneumonia and emphysema Monday at the Southern Maryland Hospital Center in Clinton. He had lived in Bryans Roads since 1965.

Mr. Flack retired from the Hagner firm in 1963 after about 25 years of service there. Earlier, he had worked in the insurance department of the H. L. Rust Co., also a Washington real estate firm.

He was born in Liverpool, England, and moved to this country as a child with his family, which settled in Pennsylvania. He came to the Washington area while in his teens.

During World War I, Mr. Flack served in France with the Army. He returned here after the war and earned a law degree from Georgetown University. During World War II, he was an air raid warden in Chevy Chase.

Mr. Flack was a member of the American Legion. He belonged to St. Francis Episcopal Church in Potomac.

He is survived by his wife of 55 years, Mary Dorothy Flack of Bryans Road; two daughters, Betty F. Bridgman of Potomac and Alice Patricia Smith of Bryans Road, and four grandchildren.