Mensa was founded in England in 1946 by Roland Berrill, a barrister, and Dr. Lance Ware, a scientist and lawyer. The original aims were, as they are today, to create a society that is non-political and free from all racial or religious distinctions. Mensa welcomes bright people from every walk of life, with the objective of enjoying each other's company and participating in a wide range of social and cultural activities.
The first handful of American Mensa members joined between 1951 and 1959. They were mostly expatriated Britons or Americans who had learned about Mensa while visiting England. One such American was a reporter named John Wilcock, who attended a Mensa meeting while visiting England. He returned and wrote a column about Mensa for The Village Voice. Peter A. Sturgeon, a medical writer in Brooklyn, fortuitously read that article, wrote to the Mensa Selection Agency on Mar. 8, 1960, and became a member as of May 1960.
There is a short YouTube video about the history of North American Mensa, which you can view by following the link below.
History of North American Mensa (U.S.)