That depends a bit based on how your terms "mental" and "governor", so I'll take my first 2 views on it.
If by mental we mean in the microcosmic sense of my own mental activity; and governor meaning one who has rightful domain to use a thing;
Then I answer: I am. I am the rightful ruler/king/governor of my own thoughts, emotions, and actions. Where there is ownership, there is responsibility and consequence as well, etc
Since I have been reading a lot of hermetic literature lately: I could also interpret "mental" in the wider macrocosmic sense: like in the first hermetic principle of the Kybalion: (all is mental). Perhaps our reality and everything we experience here is actually mental in nature; in the mind of the All. The governor in this case would be the creator. In a pretty deist hermetic/ rosacrucian sort of way
Who is the mental governor?
That depends a bit based on how your terms "mental" and "governor", so I'll take my first 2 views on it.
Thanks for the thoughtful answer. If there's only one person on the planet, do you need / have a govern-ment?