Fields worked at the Sierra Club for 16 years before retiring in 2023. She started as their National EJ Director in 2007, serving ten years before being promoted to Senior Director of Environmental Justice and Healthy Communities. In 2020, she was promoted to National Director of Policy Advocacy and Legal, overseeing the organization's lobbying, policy, and legal teams. She also led a team of more than 80 staff, including policy experts, federal and state lobbyists, national campaigners, and attorneys.

Her expertise has won her numerous awards, including the Mike McCloskey Award from the Sierra Club, the Dedication to Diversity and Justice Award from the American Bar Association Section on Environment, Energy, and Resources, the Gertrude E. Rush Distinguished Service Award from the National Bar Association, and the Black Women Green Future Award from the New Voices for Reproductive Justice.

She serves on the board of Adeso African Solutions, an East African nature resources and development organization in Kenya. She is an adjunct law professor at Howard University School of Law. Fields was previously appointed by President Obama to serve on the Board of Directors of the Mickey Leland Urban Air Toxics Research Center, highlighting her expertise in local, state, federal, and international environmental justice, law, and policy. She is licensed in the Commonwealth of Massachusetts, the District of Columbia, and the U.S. Supreme Court.