John was originally the leader, he had been the oldest (pre-Ringo) and the others looked up to him. Paul (who had always led them in the studio) started exerting more and more influence around 1965 (when John started getting depressed and into drugs) and really cemented his leadership by 1967 with Sgt. Pepper and MMT. When the latter project failed, Lennon got one last hurrah with the White Album, which is a lot rougher - like an anti-Pepper. After that, he stopped caring, while Paul tried to keep the band together.
In short, John was the original leader, Paul took over ...