Active in professional associations since 1964 when she joined the Special Libraries Association (SLA) and two years later the Geoscience Information Society (GIS), Hallmark has participated in numerous national and international committees, projects, and conferences. Her positions have included a term as president of the GIS, at the same time serving as a member of the Board of Directors of the American Geological Institute. Her work over the years in the geosciences focused on the bibliographic control of geologic guidebooks and the establishment of the international conferences in geoscience information.

She was the first chairperson of the Texas Chapter of the American Society for Information Science (ASIS) and in 1975 was instrumental in founding the first UT-Austin ASIS Student Chapter, serving as faculty sponsor of that group for its first two years. She served on the Board of Directors of the Special Libraries Association (SLA), as chair of the Texas Chapter, and chair of the Science-Technology Division. She has been faculty sponsor of the UT-Austin student chapter of SLA for more than 20 years. She is a life member of the Texas Library Association.