Whitmarsh was later elected as a director of the Federal Reserve Bank of New York, serving eight years in the role.

Whitmarsh was the member of corporate boards including the Irving National Bank, Irving Trust Company, Famous Players–Lasky, Greenwich Savings Bank, Audley Clarke Company, Straclar Holding Corporation, New York Mercantile Exchange, and the United States Chamber of Commerce.

[11][12][13] Whitmarsh was also a close acquaintance of Swami Vivekananda,[14] and drew up a trust fund for handling the administration and proceeds from his book sales.

[18] In June 1917, Whitmarsh was appointed as the chief of the Distribution Division of the newly formed United States Food Administration.

[33][34][35] That summer, Whitmarsh spent two months in Europe working on post-war food provision issues.

[40] In 1918, King Albert I of Belgium awarded Whitmarsh as an Officer of the Order of the Crown for his work in Europe.

[5] Whitmarsh's honorary pallbearers included George L. Harrison, Gates W. McGarrah, Walter C. Teagle, and Owen D. Young.