Mother got a job in my workplace behind my back.

Don't quit without another job. Don't ask your boss or HR why they've hired your mom. I also think you should not give your work place a "her or me" ultimatum, cause they are most likely to fire you both. I would not pre-emptively talk to people about it either, with the exception of your immediate supervisor. I would mention to your immediate supervisor that you've worked with your mother before and found she was not good at maintaining appropriate professional boundaries and ask how your supervisor would like you to handle any difficulties, should they arise. In this conversation, downplay the drama as much as you can, minimize the disruption you're expecting, and focus on how your supervisor would like you to handle disruptions. If she comes into your office or by your desk while you are at work, do not engage her, firmly state "Ms. Jones (NOT "MOM"), I am busy at the moment and cannot talk. Please see [name of her supervisor] if you need help with something." Then look back at your work and don't respond. If she routinely interrupts you, go to your boss and say "Hey, can you have [my mother's supervisor] talk to her about how it's not appropriate to be coming by my desk during work hours?" If she gossips, and someone mentions it to you, say something neutral and noncommittal. If pressed for a response, state firmly that you don't engage in office gossip. Although it's fine to say to people--when they mention it to you that you know it's an unusual situation, but that you hope they will continue to treat you in a professional manner, based only upon your own work and behavior, do not drag your co-workers between you and your mother. If you fear people treating you like a kid, or assuming you're inexperienced, warning people about your mom's gossiping before she starts working or talking to them about how they should not think of you as a "momma's kid" beforehand is a surefire way to get pegged as a drama llama in someone's eyes. In the meantime, start ordering your life so that you can move out, move on to a different job and start moving on from a life that is routinely sabotaged by your mother.

If it is a small, close-knit company that you've been working at for three years, do you have the leverage to say "she goes or I do?"

You need to quietly plan your escape from this person. If you have to finish school or whatever, do that, but for your own health and welfare have A Plan. Eventually, you need to move far away AND quit your job. Or, you can continue to go " halfsies" in your boundary setting and this woman will continue to ruin your life. Do you want a happy marriage at some point? Emotionally healthy children? You can only get that if you move far away, get therapy, and maintain boundaries (as in, maybe your mother knows what city you live in, but not your address. Certainly, you can never ever share where you work with her ever again.) I'm sorry this is happening. Your mother is... not well.... and the kindest thing you can do for her and for yourself under these circumstances is engineer your life such that she can not harm you. Ask me how I know.

I'd definitely talk to whoever hired her/has the power to let her go and say something along these lines: "I want to talk to you about your new employee XX who happens to be my mother. I actually left my previous position because she also worked there and couldn't maintain professional boundaries; she has now applied here without my knowledge and has already begun exhibiting unprofessional behavior with me and expecting it from me in return. I am asking that you move her to a completely unrelated sector and/or let her go given that she either hid or falsified information during her hiring process (presumably she either didn't tell them you were her child or told them but didn't share that you don't want to work with her)." Also, were I in your position, I would move out.

Oh! In the meantime, I want you to pretend you think everything is lovely, I want you to put a superhuman effort into ignoring EVERYTHING she does to get your goat. Develop hobbies and interests that take you out of the home. Be professional at work. Put your focus on YOURSELF and not on her. No. Matter. What. Also, find a therapist. Someone who will help you work your Plan. Someone who will provide reality checks when you need them. Sadly, you will need a lot of reality checks as you navigate this time in your life. Good luck. You can do this. You can.

Why not set up an appointment with your boss and HR, and ask them why they have chosen to hire your mother? Tell them it impacts performance and moral to work alongside your mother, and you do not understand why they think it's a good idea for productivity to hire family members together. Frankly, you are in a toxic situation. It's good you're asking for help here... But I have a feeling you already know what needs to be done, but just lack the gall to do it.

You have a part time job with good wages and benefits while completing your undergraduate degree? If you are located in the US, then NO DO NOT QUIT. Suck it up, and talk to your manager about what her expectations are for the working relationship and conflict resolution. When you graduate and move into full-time employment, move out.

Did they know she was your Mother? I'm still having a hard time imagining a company saying "Hey lets hire this emplyees Mom and not tell her, it will be a nice surprise". Many companies won't hire family members to work together for all sorts of very valid reasons, yours being one of them. Usually if you know someone who already works in the company you would use them as a reference. It should have sent up all kinds of red flags that a Mother wouldn't use her own Daughter as a reference. I definitely think you should talk to someone and find a diplomatic way to say "Are you out of your effing minds?!" Have someone help you write a script that sounds businesslike but seriously this is just insane... I haven't worked in an office atmosphere for quite awhile so maybe things have changed so radically that this question makes me feel like I've stepped into the Twilight Zone but, jeeze Louise, your MOTHER?

You need to meet with your boss and/or HR and get a better understanding of the situation. The key question you need answered at this point is whether or not they knew she was your mother. In many companies, it is against policy for family members to work together, due to the possibility of nepotism or other issues that would raise concern or conflict among non-family team members. If this is the case in your company, then the issues around your existing relationship issues and conflicts are largely irrelevant.