Foster was appointed to the faculty at California State University, Los Angeles in 2000. That year she took part in Alert 2000, an international fieldwork programme to study the photochemistry of snow in Alert, Nunavut. Her research proposal looked to measure the concentrations of gaseous halogens using atmospheric-pressure chemical ionization. She has said that she joined Cal State LA because it valued teaching as much as research. There she has studied the impact of sunlight on pollutants, primarily at the air/water interface. She was awarded tenure in 2006 and the California State University, Los Angeles Distinguished Women Award in 2007. Beyond her work on pollutants, Foster has studied how phosphorus might have first been incorporated into living cells.
Foster has worked to make science and technology more inclusive and more welcoming to students of colour. She established the Cal State Minorities Opportunities in Research programme, which introduces students from marginalised backgrounds to research methods. The National Science Foundation named Cal State LA as one of the top institutions for Latin Americans. She made use of the American Chemical Society Project SEED initiative to host Black chemists in her research lab. For her efforts, she was named a Minority Access National Role Model. She is the Director of the Minority Biomedical Research Support-Research Initiative for Scientific Enhancement) (MBBRS-RISE) programme, which supports students of colour who look to become research scientists.