MARY G. ROEBLING, 89, the first North American woman to serve as president of a major banking institution, died Tuesday in Trenton, N.J., of renal failure. Mrs. Roebling, friend to numerous entertainers and politicians, including President Richard M. Nixon, broke numerous gender barriers in an illustrious career that spanned five decades. She was the first woman governor of the American Stock Exchange. Mrs. Roebling was also a founder in 1978 of the Women's Bank of Denver, the first nationally chartered bank founded by women. Her second husband was banking and steel cable magnate Siegfried Roebling, whose company, Roebling Steel Corp., built the Brooklyn Bridge.