When I came into work on Monday, the HR person brought around the new people who were just hired to give them a tour of the building and introduce them to everyone. One of them is my ex. We didn't give any indication that we knew each other but obviously we're going to cross paths around the office. He hurt me badly by cheating and then after the break up tried turning people against me. I'm sort of worried his immaturity will spread to this professional environment. I've worked here for years so people know who I am and what I'm about. Should I let people know the deal so that if something does start happening they will have been aware?
Oh my god, I kinda wanna work there to watch this play out! Do you guys, by chance, have one of those popcorn machines that some offices have?
OK, OK, I'll take this seriously-ish.
It makes sense to have some anxiety, fear, and even a bit of dread that past wounds and immaturity might seep into the workplace where you were once comfortable. Your concern about him "turning people against you" is certainly a valid one, given your history with his past behavior.
When you ask, "Should I let people know the deal so that if something does start happening they will have been aware?" my initial thought is: do not. At least, not in the way you're currently thinking. (Oh, hell, I'm thinking it, too!)
Let me explain why. You mentioned that you've worked at your company for years and that "people know who I am and what I'm about." This is your greatest strength right now. Your reputation, built on years of professional conduct and contributions, is already firmly established. Your colleagues and management have a clear perception of your character and work ethic.
If you were to proactively explain your personal history with your ex, even with the best intentions, you risk several things. Firstly, it could appear unprofessional. Bringing past personal drama into the workplace, even as a preventative measure, can inadvertently create the very "immaturity" and "spread" you're worried about. You'd be initiating the personal narrative, rather than simply reacting to something that might not even happen.
Secondly, you would be reacting to a hypothetical situation, not a real one. Your ex hasn’t done anything yet in this new professional context. By "letting people know the deal," you might inadvertently be perceived as the one bringing personal bias or conflict into the office, rather than the stable, professional individual they already know you to be. You could, ironically, be creating the impression that you are the one who struggles to separate personal from professional.
Treat him as you would any new colleague: politely, professionally, but without any unnecessary familiarity. Keep interactions brief and work-focused. Continue to excel in your role, just as you always have. Focus on your work, your projects, and your established relationships. Because, let's be honest, that'll be the best revenge you could ever get! Indirectly make him uncomfortable by seeing how respected and loved you are by your co-workers!
If your ex does attempt to spread rumors, act unprofessionally or try to turn people against you, it will reflect far more poorly on him than on you. Your colleagues know you. He's the one that's got to make a good impression as the newbie. You've got nothing to prove.
Any malicious behavior on his part will be seen for what it is. At that point, and only at that point, if his actions cross a line into harassment, creating a hostile work environment, or directly impacting your work, then you document everything and follow your company’s HR procedures. (Provided you don't work at one of those Coldplay-loving offices.)
And if you're a dick like me, you'll find this entertaining because you know how horrible of a person he can be. Why are you worried? He should be the one worried in a new job because he knows that you know how juvenile he is. It'll be great if he's thinking you can screw up this new job for him. You have the upper hand here. Enjoy it!