Mark Sullivan Jr., 74, a former member of the old D.C. Board of Commissioners who had been a Washington investment counselor and broker since the 1930s and who had been a leader in civic and volunteer organizations for many years, died May 2 at Georgetown University Hospital where he was being treated for a heart ailment.

At the time of his death, Mr. Sullivan was an investment adviser with Thomson McKinnon Securities Inc. He served on the D.C. Board of Commissioners from September 1960 to March 1961. He served as president of the National Capital Area United Givers Fund in 1962 and 1963, was a director of the Greater Washington Board of Trade from 1963 to 1969 and director of the Washington Center for Metropolitan Studies from 1962 to 1972.

He served on the D.C. Republican Central Committee and as head of the D.C. Republican Platform Committee in 1964.

Mr. Sullivan was general partner in the Washington office of the Auchincloss, Parker & Redpath brokerage firm when President Dwight D. Eisenhower asked him to become a District Commissioner.

Mr. Sullivan accepted the appointment, but said he would step down if a Democrat was elected president in 1960. After joining the board, he served for a time as its acting president. During his months in office, he called for reform of the city's budget procedures, increases in school construction, welfare reform, and for the right of the city to send elected representatives to both houses of Congress.

Before leaving the board, he cautioned that the city's taxes and many of its problems were growing, but lauded the city's workers as being as capable and dedicated as any in the country. Upon stepping down, he announced that his first order of business before rejoining his brokerage was to take a vacation. He said he had planned to go fishing for a long time, but did not feel quite right about taking a vacation while on the city payroll. He was replaced on the board by Walter N. Tobriner.

The three-member Board of Commissioners later was replaced by an appointed mayor of the District of Columbia, who, in turn, was replaced by an elected mayor.

After leaving the board of commissioners, Mr. Sullivan continued his civic and volunteer work. Later in the 1960s, he served as president of the Visiting Nurse Association, a director of the National Capital Area Health and Welfare Council, and as a trustee of the Federal City Council, the United Planning Organization and George Washington University. From 1969 to 1972 he was a member of the D.C. Board of Library Trustees.

Before becoming a District Commissioner, he served on the budget steering committee of the Health and Welfare Council, was a finance chairman of the Commissioners' Public Works Review Committee, and spent several years on the Commissioners' Citizens Advisory Council.

Mr. Sullivan was born in New York City and moved here at an early age. His father, Mark Sullivan, was a noted historian and newspaper columnist with the New York Herald Tribune. Mark Sullivan Jr. attended Sidwell Friends School and graduated from the Lawrenceville School in New Jersey. He was a 1932 graduate of Princeton University, where he was a member of Phi Beta Kappa. During World War II, he became flag secretary to the head of Navy Public Information and attained the rank of lieutenant commander.

He began his business career in 1933 as an investment analyst with Auchincloss, Parker & Redpath, and from 1937 to 1950 was comanager of its investment advisory department. He was general partner in charge of the brokerage's Washington office in 1970 when it merged to become what is now Thomson McKinnon Securities Inc.

Mr. Sullivan was a past president of both the Princeton Club of Washington and the Chevy Chase Club and a former governor of the Metropolitan Club in Washington. He served as board chairman of the Beavoir School in Washington.

His wife, the former Martha S. Davidge, whom he married in 1938, died in 1978. His survivors include three sons, Mark III, of Bethesda, John D., of Nantucket, Mass., and Richard B., of San Rafael, Calif.; a sister, Narcissa Seigchrist of Randolph, Vt., and three grandchildren.