I've always had a mentor.
At this very moment, we're a pretty awesome team at Hashnode and I consider all my colleagues my mentors and try to pick up as much as I can from them. I also try to pass along whatever I know to them as well, but I would say that the former is more dominant than the later.
At my first job, I had two mentors. They taught me the basics of writing clean code.
At my second job, I probably had the best mentors I could've asked for. Tough love at its very best. The code reviews were rather aggressive to the extent that you would want to cry yourself to sleep in the night, but those reviews really made me a better programmer. If you've seen the movie Whiplash, you would know this line.
There are no two words in the English language more harmful than "good job".
That's how we rolled there.