環境法
Before politics, Duncan founded and ran the Environmental law Center and practiced as an environmental lawyer, working in Edmonton until 1987 when she moved to Ottawa to work for Environment Canada. She then taught environmental law at Dalhousie Law School (now the Schulich School of Law) and advised the Government of Indonesia on environmental assessment and enforcement. She also spent time in Whitehorse working as an assistant deputy in the Yukon government, later consulting with Kluane First Nation and later in Montreal as Head Law and Enforcement for the NAFTA's Commission for Environmental Cooperation. She also served on the Sierra Legal Defence Fund (now ecojustice)Board of Directors.