The only example that comes to mind was Ramanujan, who made many new contributions in number theory, but it's not quite the same thing -- though his genius was unparalleled, he was also working on domains that were at the time not as widely studied as nuclear fusion is today.
Cubic equations were researched by mathematicians for thousands of years before Tartaglia solved the general case! (btw the solution for Quadratic equations was well known since at least 2000 BC)
And he also made up the complex numbers on the way!
Wrt whether he was an outsider, http://en.wikipedia.org/wiki/Niccol%C3%B2_Fontana_Tartaglia
> There is a story that Tartaglia learned only half the alphabet from a private tutor before funds ran out, and he had to learn the rest for himself. Be that as it may, he was essentially self-taught. He and his contemporaries, working outside the academies, were responsible for the spread of classic works in modern languages among the educated middle class.
After Tartaglia's solution for Cubic equations and Ferrari's solution for Quartic equations were published in 1545, no doubt that finding a solution for 5th degree polynomials became a hot topic. http://en.wikipedia.org/wiki/Quintic_function
> Finding the roots of a given polynomial has been a prominent mathematical problem.
But even though it was a hot topic, it took 300 years until Galois came around with a method to determine which Quintic equations can and which cannot be factored to "radicals".
Einstein, of course.
Einstein: physics-trained, in Switzerland, married a physics classmate, learned electromagnetism from his father and uncle who were in the power generation business, taught physics, worked in the patent office, certainly a good place to be exposed to the froth of new ideas. Read the Isaacson biography.
Einstein, in 1905's "miracle year", was pretty much outside of the physics establishment (why he worked in the patent office).