I did not earn my Master's degree. Do I mention it in my Ph.D. program and onwards and on my CV? I started my terminal Master’s degree program in Fall 2017. My plan was to graduate Spring 2019, and if successful, enter a Ph.D. program  Fall 2019. My Master’s program is a more general program and not one that is necessary to obtain in order to proceed to my specific Ph.D. program, however, some like myself, use it as a stepping stone especially if we come from other fields (I hold a B.S. in biology which is not my current field).

In short, I did not obtain my Master’s. Due to the fact that I was immersed in a highly toxic, dysfunctional (no one has integrity and conducts unethical research) and borderline abusive department, and that both my thesis chairs left in the process, my graduation date was delayed. There were also 3 others in my cohort who didn’t graduate, and many others dropped out along the way—all for similar reasons as I. I was accepted to my top choice Ph.D. program in a different state, and thus decided to not stay behind and try to complete my Master’s (which was my back up plan). My new school does not expect me to complete my Master’s; in fact, I’d almost say that they don’t care, as they did not ask 1 single question about my program nor seemed remotely interested about it during my interview. Not needing my Master’s was confirmed by both the Admissions Department and the Records Office at my new school, and after this, I formally withdrew from my former school. My experience was so negative that I have zero plans to return to complete the Master’s at a different time. Also, at my new school, you earn a Master’s (which is in my specific field and credit based) along the way, which is nice.

My question is, is it necessary to mention my Master’s program on my CV? I’m assuming the answer is ‘yes,’ but I am unsure of how I would go about mentioning it on my CV. I was 3 credits away from getting my Master’s, and while I’m not ashamed of it, I am unsure of how I should handle it when it comes up in convos, esp. when applying to internships, etc. Do I say “degree not conferred” or is there a better way to convey this? 

I’d appreciate any tips and advice. Thank you!
You don't need to emphasize or even mention such things unless asked to account for the time you spent. In general, it is a poor choice to mention things you think are failures. In any application you want to emphasize your suitability for the position and the high likelihood that you will be a success at it. Stress the positive. 

But be prepared to give an honest and sensible answer if asked what you were doing for a period of time longer than a few months. It is probably fine to say that you pursued a dead end and changed course, which sounds accurate. It is also fine to say that you escaped a dysfunctional environment after being caught up in it for a while. But that isn't something for your CV or your SOP. 

You will probably be asked to supply transcripts of all academic programs in some applications. You need to do that, of course. But you don't need to characterize the situation as a failure unless the institution considered it so.