What is the meaning of "one must of necessity"?

Contrary to a comment, OP's example is a perfectly reasonable sentence. It's attributed to James Thomas Fields, apparently first published 1900 by Modern eloquence (they thought it was okay!).

The component of necessity is just an intensifier of must - some might say it's tautological, but I see nothing wrong with that (or 15,000 more instances of "must of necessity" in Google Books).

The sentence says that a great leader is always master of the situation - that's why "so" links the two descriptions, because the first implies/entails/necessitates the second. But it also says you definitely can't be a great leader unless you've been a "great thinker in action".

The term must can mean you are under an obligation imposed by some authority:

"You must go to school!"

"Sez who?"

In the alternative, it can mean that something need be done because of the intrinsic nature of the situation:

"To set an object in motion, you must introduce an outside force."

In the quoted sentence, of necessity suggests it is the latter type of must.

"One must of necessity..." = "It is necessary that one..."