John Forbes Nash (1928-2015) had insights into a branch of mathematics known as game theory. His chief contribution, which he humbly described as his "most trivial," was the Nash equilibrium that won for him the Nobel Prize in Economics in 1994. His Nobel Prize was based on his 27-page dissertation that he wrote at the age of 21, entitled "Non-Cooperative Games" (1950).
While merely an undergraduate, Nash proved Brouwer's fixed point theorem.
His career was popularized by the book by Sylvia Nasar entitled A Beautiful Mind, and by the movie of the same name.