I never had a mentor. Once a boss promised to teach me how to manage people, but then she promptly disappeared to "work from home" for weeks on end and was never seen again.
What I had instead were anti-mentors: bosses who were so bad that they unwittingly formed the start of my thinking on management, by providing a perfect model of what not to do.
My first boss was so afraid of offending anyone or making waves that he stood idly by while the organization crumbled around him. About half the staff there did little to no work, and he said nothing about it. He would sometimes complain about people behind their back but he never addressed anything to anyone's face. It was impossible to get warned about anything, let alone fired. One coworker and I used to speculate on how outrageous someone's behavior would have to be before he would be forced to say something to them. At one point, we decided that I could come to work wrapped in a bath towel, as if I'd just stepped out of the shower, and he wouldn't comment on it. We resorted to begging the higher-ups to hire a real manager, but our pleas went nowhere and we eventually left.
Later, I had another boss who openly talked about how she hadn't wanted the promotion that had made her the manager of our department, and it was clear that her strategy was to pretend nothing had changed. Requests from other departments for work from us would sit in her in-box for days because she either didn't want or didn't know how to assign work. Eventually the department that had sent the request would call to check on it, at which point she would assign it to someone who would be forced to drop everything to complete it at the last minute. A co-worker and I used to devise ways to get work done despite her; at one point we installed a work order box outside the department and announced that all incoming jobs had to be requested via a form left in the box, so we could just grab jobs and do them, before they got bottlenecked with our alleged "manager."
I had another boss who brought me in to "fix" problems on the staff and who loved to sit in his office and complain to me about how those problem staffers were holding the organization back. Ironically, he also loved giving flowery speeches about the importance of strong management -- until I told him it was time to start holding those problem staffers accountable and insisting they start getting some results. Then he filibustered for months, coming up with one reason after another why we couldn't take any action, until I finally realized he would never bring himself to make waves. Many years later, long after I left in frustration at his inaction, those problem staffers are still there, their problem behaviors unchanged.
I could go on and on. But the point is this: My bad bosses taught me what eventually became the foundation of my own approach to management, by teaching me what not to do. Once you know what not to do, the path to what you should do becomes remarkably clear.
By working for managers who allowed their desire to be nice to lead them to avoid unpopular/difficult decisions and conversations, I learned how crucial it is to address problems straightforwardly. By working for managers who tolerated shoddy work, I learned the importance of setting a clear and high bar and expecting people to meet it. By working with managers who didn't know how to delegate, I learned how key it is to be hands-on in keeping work moving, including laying out clear expectations about results, checking in on progress, and holding people accountable for their performance. And from various other bad managers, I learned to see and use authority as just one more tool in the toolbox for getting things done; it's not something that should make you nervous or something to lord over others, just something that helps you run things in the way they should be run, and to back up your words with action.
And now that I manage other managers, I make damn sure none of them are going to be the nightmare manager that someone else is writing about someday.
So here's a shout-out to all the bad managers from my past. You put me on the path to my current job and, in the words of the terrible Chicago ballad, you're the inspiration. Thank you!