G. Spencer-Brown (April 2, 1923) was born in Grimsby, Lincolnshire, England and is a British mathematician. His full name is George Spencer-Brown, but he published using just his first initial "G".
Spencer-Brown is best known for his book Laws of Form. The calculus presented in this book has been very influential to some (e.g. Heinz von Foerster, Niklas Luhmann, Francisco Varela), but others see it as incomprehensible and tautologic. Spencer-Brown claimed to have proven the four-colour theorem using his calculus of form. He has a patent on an elevator controller.
He has an M.B. of the University of London (1940) and studied later at Trinity College, Cambridge (1947-1952), where he received a diploma of education. From 1952 to 1958 he was at Christ Church, Oxford, where he worked about statics and probability, the later the theme of his PhD thesis. In 1964 he became a lecturer in formal mathematics at the University of London.
Spencer-Brown is described on the Webpage LawsOForm as "mathematician, consulting engineer, psychologist, educational consultant and practitioner, consulting psychotherapist, author, and poet"  (http://www.lawsofform.org/gsb/vita.html); he worked in a number of positions. From 1969 onward he was a Member of the Department of Pure Mathematics and Mathematical Statistics of the University of Cambridge. In the 1970s and 1980s he was visiting professor to the University of Western Australia, to Stanford University and to the University of Maryland.
1977 he married "by special contract" (as the website Laws Of Form says) Katherine Lynn Parker; they later separated.