Who took the first presidential oath of office under the U.S. Constitution?

George Washington took the first presidential oath of office under the U.S. Constitution.

Washington's inauguration took place on April 30, 1789, at Federal Hall in New York City, then the nation's capital. Chancellor Robert Livingston of New York administered the oath on the building's second-floor balcony before a large public crowd. Washington then delivered his First Inaugural Address to Congress.

The constitutional government had technically begun operating on March 4, 1789, but logistical delays prevented the Electoral College votes from being counted and certified immediately. Congress achieved a quorum in April, confirmed Washington's election, and arranged the ceremony.

John Adams became the first vice president, assuming that office before Washington's inauguration. The event therefore marked the start of the executive branch under the new Constitution, not the beginning of the earlier Revolutionary-era governments established by the Continental Congress and the Articles of Confederation.