National Security Agency
The National Security Agency is a branch of the United States government (specifically, of the US Department of Defense). Its primary mission is information security and signals intelligence, or, more simply put, cryptography.
The NSA is likely to be the largest employer of mathematicians in the United States. (We say "likely" because the exact number of mathematicians employed at NSA is classified.)
The NSA supports the United States of America Mathematical Talent Search, a mathematics competition for high school students administered by the Art of Problem Solving Foundation.