I believe I'd read that he also achieved most of his best results before espousing his "less conventional" ideas later.
I hear this all the time but not only is it not true but it is verifiably untrue. First of all, he was inspired when he first saw Casey Viator. It was through Casey that Mike learned about Arthur Jones. Mike wrote that this was at a time when he was growing very unsatisfied and uninspired with his progress. After speaking at length with Jones it gave Mike, like so many others, including myself, who took the current training six days a week, 20 sets/body part, 2 hours/day as gospel because, after all, that was what all the Arnold era champs were doing; a whole new perspective on training.
People take it for granted that back in those days, for me that would be the 1970s, nobody talked about the role of genetics and the importance of recovery. It was always about the pump and training more. If you had a "weak body part" the solution was always to do more sets. For someone like me, and the vast majority of average Joes out there, EVERY bodypart was weak so that left us in quite a dilemma. Training more and more and more. Sure everybody knew we all needed a good night's sleep but never was the concept that results occur outside the gym during the recovery period. Jones made the point that the solution is rarely to train more but to train harder. The one concept that stood out for me, and Mike Mentzer, simply because it made logical sense, was that: As long as you are working within your functional ability. As long as you are doing things that are already easy. Exercise will do little or nothing by way of increasing, size, strength, and functional ability. If you can do 10 pull-ups and just keep doing 10 reps never even attempting the 11th rep, you will not stimulate an adaptive response. Isn't the whole point of lifting weights was to subject your body to a stimulous that it was not accustom to?
When you look at Mike when he first move to Florida to train with Jones you can see he was nowhere near the level of developent he was later to achieve using the basic, and somewhat tweaked, principles he learned from Jones.