User blog comment:Miskos3/Fairy Tail Episode 250: Tartaros arc: Erza vs. Minerva, Review/@comment-24277053-20150912120428/@comment-7418318-20150912134843

It was never mentioned when Jiemma became Master. They only said that Sabertooth became famous due to a couple of strong members joining.

It could well be that Jiemma was Master all along, but was unable to turn Sabertooth into what he wanted them to become until after he managed to hammer Minerva into shape, and use her strength (and Fairy Tail's absence) to successfully attract the attention of the other strong members.

With his mindset he might well have refused to officially let Minerva join his guild until she proofed herself to him.