Bloomberg Philanthropies · CC0

Was fired from Salomon Brothers in 1981 and used the severance to found the company that became Bloomberg L.P., building a financial-data terminal that made him indispensable to Wall Street trading desks years before he entered politics. He served three terms as mayor of New York City starting in 2002, switching party affiliation twice along the way, and has since given away tens of billions of dollars to public health, education and climate causes through his foundation — philanthropy on a scale that reshaped what a mayor's post-office career could look like.