She began her career as a postdoctoral fellow at the Woods Hole Oceanographic Institution where she studied copepod dormancy and its implications for marine aquaculture.

She continued her field research as a professor of oceanography and later as the director of the Florida State University Marine Laboratory (FSU).

She served on the Goucher College Board of Trustees from 2009 to 2016 and was previously the chair of the academic affairs committee.

[2] In 1971, Marcus was on a team of undergraduate researchers from Goucher and Towson University who participated in a study entitled "An Analysis on the Degradation of Lake Roland."

[5] In her senior year, Marcus and a colleague conducted field research on phytoplankton at the Duke University Marine Lab in January 1972.

[9] In 1978, Marcus was hired as an associate scientist in the biology department at the Woods Hole Oceanographic Institution.

[9] In 1984, Marcus and associate scientist George Grice completed a two and a half week trip in China at the invitation of professor Zheng Zhong, the founder of the marine biology program at Xiamen University.

[10] Marcus began working at Florida State University (FSU) as an associate professor of oceanography in 1987.

[7] In the summer of 2001, Marcus issued a joint comment with professors Patricia Yancey Martin, Jean G. Bryant, Dianne F. Harrison, Gary R. Heald, Sheila Ortiz-Taylor, Pamela L. Perrewé, and David W. Rasmussen on status of women in academia the biases and "subtle, non-conscious forms of discrimination" that persist.

[17] Her later research investigated the impacts of decreased oxygen levels on the population dynamics of marine copepods.

[18] Marcus was involved in the creation of a "standardized system and management protocol for rearing marine fish larvae".

[19] She built two greenhouses at FSU,[20] which were later converted into laboratories to evaluate interactions between aquatic species and study marine plants.

[4] After serving for five years on the Council of Graduate Schools Board of Directors, Marcus was appointed Chair for the 2016 term.

Ortega also remarked that "as graduate dean, she has given particular attention to broadening career opportunities for graduates and has developed programs to support their success in a range of sectors — academia, industry and government...the CGS board of directors will benefit greatly from her leadership and expertise.

[4] From August 2017 through the end of the fall term, Marcus was the Lawton Professor in the Department of Earth, Ocean, and Atmospheric Science.

Following her death, a service was held at the FSU Alumni Center Grand Ballroom and the Nancy Marcus Endowment for Graduate Student Excellence was established.

[4] In 2014, the main auditorium of the FSU of Honors, Scholars, and Fellows House was named the Nancy H. Marcus Great Hall.