Thomas McKean Thompson McKennan (March 31, 1794 – July 9, 1852) was a 19th-century politician and lawyer who served briefly as United States Secretary of the Interior under President Millard Fillmore.

Despite immense pressure from associates, friends, and the Washington County Whig Party, McKennan again refused to run for another term in Congress, declaring that he had done his duty by serving in public office, and it was time to return to Washington, Pennsylvania, and focus on his law practice.

[4] Contemporary accounts also cite disagreements with President Fillmore and Secretary of State Daniel Webster regarding the appointment and removal of attorneys, marshals, clerks, and heads of bureaus.

[5] During his brief time as Secretary, McKennan was the head of the 1850 Census, which was being conducted that summer, and he issued a remarkably foresighted statement on the importance of protecting individual privacy: Information has been received at this office that in some cases unnecessary exposure has been made by the assistant marshals with reference to the business and pursuits, and other facts relating to individuals, merely to gratify curiosity, or the facts applied to the private use or pecuniary advantage of the assistant, to the injury of others.

...all marshals and assistants are expected to consider the facts intrusted to them as if obtained exclusively for the use of the Government, and not to be used in any way to the gratification of curiosity, the exposure of any man's business or pursuits, or for the private emolument of the marshals or assistants, who, while employed in this service, act as the agents of the Government in the most confidential capacity.