According to The Last Jedi novelization, which was written by Jason Fry with the assistance of Rian Johnson, Snoke was the unexpected leader of the First Order.
It isn’t clear when Snoke stepped up and took control or if it was right from the start, but The Last Jedi novel states that Snoke saved the First Order from many enemies.
The novelization of Rian Johnson’s The Last Jedi is already delivering on its promise of being the “extended edition.”
There are spoilers for The Last Jedi novel ahead, so read ahead at your own risk.