The Mona Lisa was sent away from the Louvre before the Nazis Advanced on France. There is some mystery as to where the Mona Lisa was sent. This is because there was also a copy of the Mona Lisa, which was sometimes displayed as the original Mona Lisa. One painting, and we do not know if it was the actual Mona Lisa or the copy, was sent to various Chateuxs across France. If it was the copy, the theory is that this was to send the Nazis on a wild goose chase. Under that theory, the actual Mona Lisa was hidden somewhere in Paris, although I know of no resource that indicates that it was. Regardless the Mona Lisa was absent from the Louvre in 1939 just before the Nazi Advance on France, and remained hidden until 1945 when the World War II was done.