Dr. L. Quincy Mumford, 78, the librarian of Congress from 1954 to 1974, when the library underwent a rapid expansion of its staff, services and facilities, died of a heart ailment Aug. 15 at his home in Washington.

Under Dr. Mumford's guidance, the library opened its first acquisition centers abroad in New Delhi and Cairo, gained possession of the famed Rosenwald Collection of 700 rare books, and renovated its main reading room. It also undertook the microfilming of the presidential papers and did studies that led to the first revision of copyright law in more than 50 years.

When Dr. Mumford retired, former president Gerald Ford told him that during his years as head of the institution "the demands on the library's facilities and staff have multiplied, and your own challenging responsibilities have similarly changed and increased. But you have unfailingly discharged your duties in a manner which has properly won the respect and admiration of your many friends in the Congress and the lasting gratitude of all your fellow citizens."

The present librarian of Congress, Pulitzer Prize-winning historian Daniel Boorstin, said after learning of Dr. Mumford's death, "When Quincy took over the library there was a staff of 1,600 and an appropriation of $9.4 million, and when I became librarian 20 years later this had grown to 4,500 employes and a $96 million appropriation.

"One of his greatest triumphs was the achievement of the James Madison Memorial Building. The introduction of automation under his leadership brought unimaginable improvements in the library's services to the Congress, the world of scholarship, and libraries."

Lawrence Quincy Mumford was born in Ayden, N.C. He earned bachelor's and master's degrees in English at Duke University. He then took a degree in library science at Columbia University. He held honorary doctorates from a number of universities, including Duke and Columbia.

From 1929 to 1945, he was a member of the staff of the New York Public Library. He then was appointed assistant director of the Cleveland Public Library. He taught library science at the old Western Reserve University from 1946 to 1954. In 1950, he was named director of the Cleveland library and held that post until he was appointed Librarian of Congress by President Dwight D. Eisenhower. He was the first person to hold that position to have a degree in library science.

Dr. Mumford had worked at the Library of Congress in 1940 and 1941, when he was on leave from the New York Public Library. During World War II, he was a member of the committee for the survey of the Army Medical Library.

He was president of the American Library Association in 1955. He was a member of the board of directors of the Films Council of America, the board of advisors of the Dumbarton Oaks Research Library and Collections and a trustee of the Great Lakes Historical Society. He was a member of the Cosmos Club.

His first wife, Permelia Stevens Mumford, died in 1961.

Survivors include his wife, Betsy Perrin Mumford, of Washington; a daughter by his first marriage, Mrs. Lawrence Deane, of Philadelphia; three stepchildren, Army Lt. Col. Barry Fox, of McLean, Mrs. Peter V. Swindler, of Fort Sumner, Md., and Mrs. Elmer C. Holt, of Fairfax, and 12 grandchildren.