President Washington appointed L'Enfant in 1791 to plan the new "Federal City" (later named the "City of Washington") under the supervision of the three Commissioners, whom Washington had appointed to oversee the planning and development of the federal territory that would later become designated the "District of Columbia". Included in the new district were the river port towns of Georgetown (formerly in Montgomery County of the State of Maryland) and Alexandria (in Fairfax County, in the Commonwealth of Virginia).
While L'Enfant was in New York City, he was initiated into Freemasonry. His initiation took place on April 17, 1789, at Holland Lodge No. 8, F & A M, which the Grand Lodge of New York F & A M had chartered in 1787. L'Enfant took only the first of three degrees offered by the Lodge and did not progress further in Freemasonry.[18]
For that matter the buildings and monument, even the layout of DC was done by Masons.
Not entirely correct. DCs original borders came into the City of Alexandria, but not so far as Fairfax County.