Mary Catherine Barlow Smallwood, 59, a retired judge who had served in D.C. Superior Court and its predecessors, the Court of General Sessions and Municipal Court, died Thursday at her home in Washington.

Death was attributed to heart disease. She had been in ill health for much of the latter part of her 21 years on the bench before she retired in 1971.

When she was appointed to the Municipal Court bench by President Truman in 1950, Judge Barlow, at 32, was the youngest judge in Washington and the youngest woman ever to be appointed to a court of record by a President of the United States.

Born in Grafton, W.Va., she was a graduate of the old Columbus University School of Law, now part of Catholic University. She also earned a degree in library science from Catholic University's Graduate School of Arts and Sciences.

From 1938 to 1941, Judge Barlow was registrar of Columbus University Junior College. She then spent a year as law librarian at the university.

From that position, she moved on to become a full-time professor of law. She was the first and at that time the only woman on the undergraduate faculty.

Judge Barlow became legal assistant to the then 10 judges of the Municipal Court in 1944. She carried the title of law librarian of the Municipal Court and held that position until her appointment to the bench.

During those years, she also taught legal bibliography in evening classes at National University Law School, now part of George Washington University. Then she also was the only woman on the law faculty.

Judge Barlow's term expired in 1960, but she remained on the bench because a successor was not named. She was reappointed to a second 10-year term in 1961 by President Kennedy.

Her attendance at court began to fall off sharply in 1965. It was blamed on illness, although details of the illness were not disclosed.

Her absence continued during the next few years and she was present in court only 30 days in 1968. There was then a continuous absence until September, 1970, when she again began presiding from the bench.

Judge Barlow declined several requests that she consider retirement and sat fairly regularly for six or seven months before her attendance again began to taper off. She then retired in September, 1971.

A member of the bars of the U.S. Supreme Court, U.S. Court of Appeals and U.S. District Court, Judge Barlow also belonged to the American, D.C. and Women's Bar Associations, the National Association of Women Lawyers and the American Judicature Society. She was a charter member of the Beta Gamma Chapter of Kappa Beta Pi, an international legal sorority.

In 1955, she married Dr. W. Lawrence Smallwood, a Washington dentist and civil leader, who survives. Also surviving is a stepdaughter, Anne Schwartz, of McLean.

The family suggests that expressions of sympathy may be in the form of contributions to the Christ Child Society.