What do you do when your Mom tells you she's planning on ending her life soon and refuses medical treatment?

What was the context in which she said this? I'm inclined to agree that if she'd made up her mind to do this thing - and some people do come to a rational decision to end their lives, after all - she'd have gone ahead and done it, God forbid, and not said anything to you about her plans. But can you say a little more about how the subject came up?

I double-checked your profile and it said your mother lives in independent living.
If your mother told you, she wants help. Really serious depression that intends self harm without intervention does NOT tell anyone. Unfortunately, I know this from personal experience in my family.
You report her to Adult Protective Services immediately. You tell authorities that she has severe depression and has told you that she intends self harm. Use those words. If she is under a professional's care for treatment of depression and you don't want to report her to authorities, tell the medical professional that she told you this. The professionals have an obligation to report it to authorities.
Not knowing the full backstory, it's hard to know if this is attention seeking or narcissism or a real depressive spiral. YOU have more than enough to handle with your daughters. Time to let the professionals take care of your mother. Holding good thoughts for you and your family in this terrible time.

My mistake she's actually living in a senior independent apartment building alone by her choice. She refuses to live with me or to have anyone check on her. I have support from my ex-husband, stepmother, his family to help with my girls, and also my mom. I also have a few close friends as well to help me cope with everything. Thank you for your suggestions.

I'm really curious if your mother has a personality disorder in addition to her depression. Is she, or has she in the past been the type of person who needs everything to be about her?

I agree with Eye, if she's in AL, call the DON. If she's in Independent Living (which is what your profile says), I would alert the director of the facility to her suicide threat. You MUST take her seriously, but you don't have to go help her yourself. Please take care of yourself. And your children.

I thought of this question right as I was posting my comment but I wonder if YOU have support. You're there for your daughters and your mom but who is there for you?

You have so much on your plate. I'm so sorry that you are going through so much.

In my opinion your mom is telling you that she's going to kill herself because she wants you to do something about it. The proverbial cry for help. I don't mean to sound insensitive and I have a great deal of compassion for people struggling with depression and other mental illnesses but you have enough to deal with right now and I wonder if your mom is just trying to get some attention from you.

Not that you shouldn't take her threat seriously. You should. Your profile says she lives in ALF? Have you let the Director of Nursing know that your mom is suicidal? That your mom has told you that she plans on killing herself and then refuses treatment she's really kind of tied your hands.

You can't make your mom seek treatment for her depression and being just one person, a daughter, there's not a lot you do but try to be compassionate when your mom talks to you and continue to encourage her to seek help.

With your daughter on hospice and having a 10 year old at home and being a single mom who's also dealing with an elderly mother how are you handling all of that?