Prior to joining the Foundation in February 2000, Mosier served in a White House appointment as director of the Office of Policy, Research, and Analysis at the National Endowment for the Arts. At the NEA, her responsibilities included advising the chairman, reviewing and recommending policies to achieve agency goals, preparing position papers, and managing agency-wide issues of major significance. She also directed analysis of the agency's application and grant award patterns and prepared statistical reports to evaluate the agency's grantmaking policies and procedures.

Immediately prior to joining the NEA, Mosier briefly worked with Keens Co. consulting on several projects for national funders such as the Ford Foundation and The Pew Charitable Trusts. She served four years as executive vice president and chief operating officer of the National Assembly of Local Arts Agencies (now Americans for the Arts), and five years as executive director of the National Artists Equity Association, both in Washington, D.C.

Mosier holds a master's degree in arts management from The American University and a bachelor of fine arts degree in art history with a minor in studio art from Howard University.