Quite some time ago, the folks at 360 asked if there have been heads of state who were by training mathematicians. This is really two questions in one: people who were trained as mathematicians, and people who had a mathematical career before going into politics.
The first question doesn't seem that interesting, because it seems to include cases in which Politician X majored in math as an undergrad, then went to law school, became a lawyer, and then entered politics from the law, as so many do. That's not the question I want to answer.
For the second question, a bit of clicking around turns up this list, which inclues Alberto Fujimori (president of Peru), Paul Painlevé (prime minister of France), and Eamon de Valera (president of Ireland). Painlevé in particular made a name for himself as a mathematician; the other two appear to have at least taught it in some capacity at some point.
I had thought that Henri Poincaré had been in politics, but it appears that I was confusing him with his cousin Raymond. Borel served in the French National Assembly. I haven't done any sort of systematic sampling, but it seems like mathematician-politicians are particularly prevalent in France, that wonderful country where they name streets after mathematicians. (Here in the United States, for example in my native city of Philadelphia, we name streets after mathematical objects, namely the positive integers.)
One interesting close call is Einstein. The story has it that he was offered the presidency of Israel in 1952. Of course Einstein was a physicist, but given the title of this blog I feel I can mention him.