George G. H. (Mac) McDaniel Jr., 57, president of the Triangle Insurance Agency in Hyattsville who was active for many years in Prince George County civic affairs, died Saturday at Prince George's General Hospital after a heart attack. He lived in Hyattsville.

Mr. McDaniel was a cofounder of the Triangle Agency in 1950 and had served as its president since then. He was a past president of the Bladensburg Rotary Club, the Prince George's Jaycees, the Prince George's Country Club and the D.C. Chapter of the 42nd Rainbow Division of the American Legion. He was former chairman of the board of directors of the Prince George's County Department of Social Services and current chairman of the Prince George's County Parking Authority.

Mr. McDaniel also was past president and secretary of the General Agents Association of the Bankers National Life Insurance Co., a member of the Prince George's Chamber of Commerce, a director of the Capital City Federal Savings and Loan Association, a charter member of the Prince George's Life Underwriters Association and a member of the Professional Insurance Agents Association and the Montgomery/Prince George's Counties Independent Agents Association.

He was born in Washington and grew up in Cottage City. During World War II, he served with the Army infantry in North Africa and Europe and received the Bronze Star. He graduated from the University of Maryland in 1950 and belonged to the "M" and Terrapin Clubs.

Mr. McDaniel was a former senior warden and member of the vestry of St. John Episcopal Church in Mount Rainier.

Survivors include his wife, Eleanor, and a son George III, both of Hyattsville, and his mother, Mary M. Houck, of Cottage City.