Final answer:
Janet Napolitano's cabinet position was the Secretary of Homeland Security, a key role in foreign and domestic policy.
Step-by-step explanation:
The cabinet position held by Janet Napolitano, for which she was the third person to serve, is Secretary of Homeland Security. The Secretary of Homeland Security is a critical member in the area of foreign policy, overseeing the Department of Homeland Security, which includes efforts to confront domestic and international terrorism. The position is one of several key cabinet roles, among others such as Secretary of State, Secretary of Defense, and Attorney General, each playing a significant part in shaping and executing the President's policy initiatives and legal mandates.