Marcum began her career teaching English at the University of Kentucky, where she switched careers to librarianship and worked for three years on a Ford Foundation program at Vanderbilt University. She then worked with the management training program at the Association of Research Libraries from 1977 to 1981. From 1980 to 1989, she worked for the Council on Library Resources. She served as Dean of the Catholic University of America School of Library and Information Science from 1989 to 1992. She served as director of public service and collection management at the Library of Congress from 1993 to 1995. In 1995, she was appointed president of the Council on Library Resources and oversaw its merger with the Commission on Preservation and Access to create the Council on Library and Information Resources. She served as president of CLIR until August 2003. From 2003 to 2011, she returned to the Library of Congress as Associate Librarian for Library Services, directing fifty-three divisions and offices with sixteen hundred employees and guiding the library toward a more digital future. Her areas of expertise included cataloging and preservation.
