The 1st United States Congress, comprising the United States Senate and the United States House of Representatives, met from March 4, 1789, to March 4, 1791, during the first two years of George Washington's presidency, first at Federal Hall in New York City and later at Congress Hall in Philadelphia. With the initial meeting of the First Congress, the United States federal government officially began operations under the new frame of government established by the 1787 Constitution. The apportionment of seats in the House of Representatives was based on the provisions of Article I, Section 2, Clause 3, of the Constitution. Both chambers had a Pro-Administration majority. Twelve articles of amendment to the Constitution were passed by this Congress and sent to the states for ratification; the ten ratified as additions to the Constitution on December 15, 1791, are collectively known as the Bill of Rights, with an additional amendment ratified more than two centuries later to become the Twenty-seventh Amendment to the United States Constitution.
The 1790–91 United States Senate elections were held on various dates in various states. These U.S. Senate elections occurred during the first midterm election cycle, which took place in the middle of President George Washington's first term. As these elections were prior to the ratification of the Seventeenth Amendment in 1913, senators were chosen by state legislatures. Senators were elected over a wide range of time throughout 1790 and 1791, and a seat may have been filled months late or remained vacant due to legislative deadlock. In these elections, terms were up for the nine senators in Class 1.