United States ex rel. McLean v. Vilas was a United States Supreme Court case that addressed the issue of whether the United States government had the authority to appoint a postmaster without the approval of the Senate. The case arose when the United States Postmaster General appointed a postmaster in the state of Wisconsin without the approval of the Senate. The postmaster, William McLean, was then removed from office by the Secretary of the Interior, Carl Schurz. McLean then filed a petition for a writ of mandamus in the Supreme Court, arguing that the Postmaster General had the authority to appoint him without the approval of the Senate. The Supreme Court ultimately held that the Postmaster General did not have the authority to appoint a postmaster without the approval of the Senate. The Court reasoned that the Post Office Department was a part of the executive branch of the government, and that the President had the power to appoint postmasters with the advice and consent of the Senate. The Court also noted that the Post Office Department was subject to the control of Congress, and that Congress had the power to regulate the appointment of postmasters. As such, the Court held that the Postmaster General did not have the authority to appoint a postmaster without the approval of the Senate.

In United States Ex Rel. McLean v. Vilas, the Supreme Court was asked to decide whether a postmaster general could be sued in his official capacity for money damages without congressional authorization. The majority opinion held that he could not, as Congress had not authorized such suits and it would be unconstitutional to do so absent explicit statutory authority from Congress. Justice Field dissented on the grounds that there were no constitutional impediments preventing an individual from suing a federal officer in their official capacity when they have acted beyond their lawful powers or duties; rather, this should be decided by common law principles of liability and tortious conduct which are applicable to all persons regardless of office or station within society. He argued that if a government officer has committed wrongs against another person then they should face legal consequences just like any other citizen - even if those consequences include monetary damages - unless otherwise prohibited by statute or constitution.