After her law school graduation and a short stint at a corporate law firm, Foster was hired by her alma mater, UC Berkeley, as Lecturer and Coordinator of Academic Support. In 1994, she began teaching at Rutgers University in Camden, New Jersey, and received tenure and was promoted to Full Professor. In 2000, Foster published her book From the Ground Up, which is also highly regarded in environmental justice and discusses social, economic, and legal structures in the context of environmental racism. In 2001, she joined Fordham University School of Law, starting as a Professor of Law, and then in 2008, Foster was appointed Associate Dean for Academic Affairs. From 2011 to 2014, Foster was Vice Dean of the Law School. While at Fordham, she co-directed the Urban Law Center and established the Urban Consortium.

Upon leaving Fordham University in 2017, Foster began her career at Georgetown University School of Law, where she currently teaches and holds a joint appointment in the McCourt School of Public Policy and the School of Law. Foster was the inaugural Associate Dean for Diversity, Equity, and Inclusion at Georgetown Law and served in this capacity from 2021 to 2022. At the university, she is the faculty advisor to the Georgetown Project on State and Local Government Policy and Law. Foster is an affiliated researcher with the Global Cities Initiative and The Earth Commons. In August 2023, she began a position as a Visiting Professor at the Columbia University Climate School.

Foster has held many positions outside of teaching on boards, committees, and advisory groups. From 2017 to 2020, she was the Chair of the Advisory Committee for the Global Parliament of Mayors. She co-chairs the Equity Work Group of the New York City Panel on Climate Change (NPCC), to which she was appointed in 2016. Foster co-directs LabGov with Christian Iaione, which focuses on innovative forms of urban governance in cities and metropolitan areas. She is also a Founding Board Member and co-editor of the Journal of Climate Resilience and Climate Justice.

Foster has received several awards for her dedication and contributions to the field, including the Georgetown Earth Commons Institute Eco Impact Award, the Georgetown Environmental Initiative Award, and the Bellagio Residency Award from the Rockefeller Foundation. Published in 2022, Foster’s book with Christian Iaione, Co-Cities: Innovative Transitions Toward Just and Self-Sustaining Communities, won the PROSE award for Architecture and Urban Planning in 2023 from the Association of American Publishers’ Professional and Scholarly Publishing Division. The book also won the American Energy Society’s Best Book on the Commons. Foster’s other scholarly works have been published in top law journals, including Yale, Harvard, Berkeley, Stanford, and Notre Dame.