Mensa was founded in Oxford in 1946 by an eccentric, flamboyant Australian and a reserved Englishman. They had met by chance on a train. The Australian was Roland Berrill, a barrister educated in England. The Englishman was Lancelot Ware, at the time a mature student at Oxford, who was later to gain a string of qualifications in science and the law.
Ware is credited with the idea of Mensa, but it was Berrill who "founded" the society in the usual sense, on 1 October 1946. Berrill supplied the start-up cash, wrote the idiosyncratic pamphlets and became Mensa'a first Secretary. He died a few years later, having recruited in total around 400 people by self-administered IQ tests.
Dr Lancelot Ware dropped out of Mensa for many years but later rejoined and was a member until his death in 2000, with the title "Fons et Origo" from his claim that the original idea was his.
In Australia, the organisation was started by a number of members of British Mensa who had moved to Australia. We date our founding to the appearance of 3 of those members on the ABC show "People 1964", in October of that year.