Marcus B. Finnegan, 51, a Washington lawyer since 1963 who was an authority in the fields of patent law and technology transfer, died Friday in St. Joseph Hospital in Houston where he was being treated for cancer.

He was a senior partner in the firm of Finnegan, Henderson, Farabow & Garrett.

Mr. Finnegan had been an adviser to the White House Council on International Economic Policy, a State Department delegate to the Inter-American Working Group on Science and the Transfer of Technology in Brazilia in 1974 to 1975, and a consultant to the joint Soviet-American working group on intellectual property.

He also had been a consultant to a number of United Nations agencies in the fields of licensing practices and antitrust matters.

He was active in professional organizations, having served as president of the Licensing Executives Society of the USA from 1973 to 1974, president of the Licensing Executives Society International in 1975, and on the boards of the American Patent Law Association.

Mr. Finnegan was a member of the American, Federal, D.C., Virginia, International, and Inter-American bar associations. He also belonged to the American, New York, and New Jersey patent law associations.

He was a member of the Patent Lawyers Club of Washington, the World Peace Through Law Center, the Society of American Law Teachers, and the International Patent and Trademark Association.

Mr. Finnegan was born in Morristown, N.J., and was a 1949 graduate of the U.S. Military Academy at West Point. He served in the Army Corps of Engineers in Korea during the conflict there and in the Judge Advocate General's Corps before attaining the rank of captain and retiring in 1959.

He was a 1955 graduate of the University of Virginia's law school and a member of the Order of the Coif.He earned a master's degree in law at George Washington University.

Mr. Finnegan had been a professorial lecturer at George Washington University's law school since 1971 where he taught a course on "Patent and Know-How Licensing."

He survived by his wife, the former Besty N. Hammer, of the home in Bethesda; three daughters, Nancy L., of Champaign, III., Susan F. Smith, of Providence, R.I., and Katharine E., of Boston; his father, George B. Finnegan Jr., of Mountain Lakes, N.J.; a sister, Dr. Dana G., of Oakland, N.J., and a brother, George B. III, of Wilton, Conn.

The family suggests that expressions of sympathy be in the form of contributions to the Stehlin Foundation for Cancer Research, 777 St. Joseph Professional Building, Houston, Tex., 77002. CAPTION: Picture, MARCUS B. FINNEGAN