Article II of the U.S. Constitution twice imposes a duty of “faithful execution” on the President, who must “take Care that the Laws be faithfully executed,” and take an oath or affirmation to “faithfully execute the Office of President.” These clauses are cited often, but their background and original meaning have never been fully explored. Courts, the executive branch, and many scholars rely on one or both clauses as support for expansive views of presidential power, for example, to go beyond standing law to defend the nation in emergencies; to withhold documents from Congress or the courts; or to refuse to fully execute statutes on grounds of unconstitutionality or for policy reasons. This Article is the first to explore the textual roots of these clauses from the time of Magna Carta and medieval England, through colonial America, and up to the original meaning in the Philadelphia Convention and ratification debates. We find that the language of “faithful execution” was for centuries before 1787 very commonly associated with the performance of public and private offices—especially those in which the officer had some control over the public fisc. “Faithful execution” language applied not only to senior government officials but also to a vast number of more ministerial officers, too. We contend that it imposed three core requirements on officeholders: (1) diligent, careful, good faith, and impartial execution of law or office; (2) a duty not to misuse an office’s funds and or take unauthorized profits; and (3) a duty not to act ultra vires, beyond the scope of one’s office. These three duties of fidelity look a lot like fiduciary duties in modern private law. This “fiduciary” reading of the original meaning of the Faithful Execution Clauses might have important implications in modern constitutional law. Our history supports readings of Article II of the Constitution, for example, that limit presidents to exercise their power in good faith, for the public interest, and not for reasons of self-dealing, self-protection, or other bad faith, personal reasons. So understood, Article II may thus place some limits on the pardon and removal authority. The history we present also supports readings of Article II that tend to subordinate presidential power to congressional direction, limiting presidential non-enforcement of statutes, and perhaps constraining agencies’ interpretation of statutes to pursue Congress’s objectives. Our conclusions undermine imperial and prerogative claims for the presidency, claims that are sometimes, in our estimation, improperly traced to dimensions of the clauses requiring the President's faithful execution.