A principal hired me at a school 10 years ago. He was only there for one year. I am now applying for a job at the school he is currently at. Should I mention he hired me years ago in a follow up email? I haven't been asked for an interview yet.
If someone hired you 10 years ago, the odds are good that they still remember you. The exception would be if you were so ordinary that they basically didn't register you (emotions help memorizing). It's unlikely.
You have zero benefit in bringing the topic up IMHO. If you had a great or terrible performance, rest assured they remember it. If you were neither and they don't remember, then at best they play along and wonder wait wtf I actually worked with this person and it was so dull I don't recollect, and at worst they get a stress response because you raised that their memory is oh so fallible and oh crap their boss at the time was just horrendous and it negatively affects you by association.
The best you can do is to build your CV so that it's clearcut you worked with them then. Perhaps also add "looking forward to working you again" if they're the recipient of your cover letter. But leave it at that. Let them do the remembering rather than rubbing it in their face.
I don't think that mentioning your old boss is the right approach when talking to an interviewer, especially if you have no idea if you will be working with that old boss.
If you will 100% be working with the old boss and he is in the hiring process then he should be able to do the math himself.
On the other hand if you think you had a good working relationship with your old boss it may be advantageous to contact him directly and tell him that you are applying for a position where he is now. At the very least this could provide you with valuable inside information about the position and hiring process.
It's not what you know .. it's who you know!