The National Museum of American History is located between 14th and 12th St. & Constitution Ave. NW.
Parking: there is limited parking on the street (free—3 hour time limit on Madison—Mall Side), and there are garages in Penn Quarter (between E & F on 12th Street, etc.).
Metro: The Museum is metro accessible, and is a 10-15 minute walk from Metro Center (Red line), and 5 minute walk from Federal Triangle (blue and orange) and Smithsonian (blue and orange). If you take the yellow or green line, it’s about a 15-20 minute walk from L’Enfant Plaza or the Archives-Navy Memorial/Penn Quarter station on 7th and Pennsylvania Ave.
Constitution Avenue is the main entrance, and the only one open after 7:30 PM.