NEW YORK — John J. Mack, president and chief operating officer of Morgan Stanley Dean Witter & Co., announced on Wednesday that he will leave the firm on March 21.

Morgan Stanley’s board of directors accepted his resignation and named Robert G. Scott, the firm’s chief financial officer, to succeed him, the investment banking company said. Mr. Scott will run the investment bank alongside Philip J. Purcell, the company’s chairman and chief executive officer.

“No one has done more than John Mack to make Morgan Stanley Dean Witter the world’s leading financial services firm,” Mr. Purcell said in a press statement.

Mr. Mack, 56, joined Morgan Stanley & Co.’s bond department in 1972 and became a managing director in 1979. He was named president in 1993, four years before the company merged with Dean Witter, Discover & Co.

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