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. She received grants from the
Gates Foundation,
Lumina Foundation, and
Mellon Foundation. She retired in 2016, succeeded by former
Vassar College president
Catharine Bond Hill, but continued to work for Ithaka S+R as a senior advisor for educational transformation and libraries and scholarly communication until her death in August 2022. In 2013, she was appointed to the
Japan-US Friendship Commission, which facilitates cultural and educational exchanges between the United States and Japan. She also served on steering committees for the
Coalition for Networked Information (2008–2012) and the
Scholarly Publishing and Academic Resources Coalition (2008–2011). ==Publications==