Matt Steinglass finds few real-world examples:
The only non-politician candidate I can think of who became a successful president was Eisenhower, and he was drafted not to clean up a mess in Washington but to end the war in Korea. Few non-politicians have been elected; in relatively modern history, only Herbert Hoover had no electoral experience, though he was hardly a Washington novice, having served two terms as commerce secretary. In any case, we all know how well he turned out. Further back, William Howard Taft had never run for anything, but he'd been in both of Teddy Roosevelt's cabinets and was hardly viewed as an "outsider", and his term was politically disastrous for the GOP. Then further back you've got General Ulysses S. Grant, whose administration was a corrupt mess. General Zachary Taylor's administration was clumsy and fortunately brief. And most non-politician white-knight candidates never make it that far.
The Dish 