I dropped out of my (CS) PhD about 6yrs ago, after having spent almost 5yrs at it. I had developed severe stress related issues, and it seemed wise to take a break. Next couple of years were spent basically trying to avoid having a total breakdown - spent a lot of time getting therapy/counselling, and traveling and spending time in Ashrams in Asia, etc. Once I got a bit better, I resumed my PhD research bit by bit (was still out of the university). I also started doing some independent research and the odd hourly consulting job. I really was not ready to strain myself, so took things slowly. Over the last year or so, I recovered my mojo and have really been able to push with my PhD research (while still on a break from the university). I just got my PhD degree finally (phew!). 

Now that I am starting to look for a full time job, I am realizing that the last 6yrs are going to show up as a big black hole on my CV. TBH, barring the first couple of years, I have been pursuing research (albeit independently for the most part). I published extensively, reviewed papers, organized conferences, and have generally been fairly active in the research circuit -  but mostly only during the last 1.5yrs or so. 

What do the job prospects for me look like? And how should I present my case to the prospective employers? 

My specialization is in Applied Machine Learning. I have led multiple research projects during my PhD and have close to 800 citations. I am looking for jobs primarily in tech companies, but am also keeping an eye out for quant and fin-tech roles.  

I would go with the truth, but not necessarily the whole truth. Basically signal that you had a serious and protracted health issue (true) and that it is no longer a problem (hopefully also true). The fact that it was a mental health thing is really not something you have to disclose. Since you happen to have qualifications in the hottest area around these days, you should have options. And once you are established somewhere your pre-doctoral history becomes less important. Good luck!

some interview pointers
Don't forget to mention that you're feeling much better now

I would go with the truth, but not necessarily the whole truth. Basically signal that you had a serious and protracted health issue (true) and that it is no longer a problem (hopefully also true). The fact that it was a mental health thing is really not something you have to disclose.

Do I need to do that even? Why not simply state that I took an extended sabbatical to travel, sort out some personal issues, and subsequently pursue other lines of research for a few years? This is precisely how it transpired, minus of course my illnesses.

Since you happen to have qualifications in the hottest area around these days, you should have options. And once you are established somewhere your pre-doctoral history becomes less important. Good luck!

Can I expect to be hired at a senior position? I am looking to wait at least another 2-3 months before sending out my job applications. What can I do in the interim to make a strong claim for a lead/senior position? This is specific to tech firms, and not quant fin firms - I don't expect anything more than an entry level position at the latter since I lack any domain experience. 

Which quant fin firms in particular might be willing to give me a break? 

My specialization is in Applied Machine Learning. I have led multiple research projects during my PhD and have close to 800 citations. I am looking for jobs primarily in tech companies, but am also keeping an eye out for quant and fin-tech roles.  

Try tech companies (Facebook, Google, DeepMind). If you have 800 citations in peer-reviewed papers, you have good chances of landing a research job there.

Is this based on first hand knowledge? They are on my radar yes, and normally people with my background shouldn't have a problem landing an interview at these places, but given my situation I am really not sure what to expect.

I just got my PhD degree finally (phew!). 
Never easy, but after a break it does show a not insignificant modicum of determination, at the very least. Good luck.

I think an explicit reference to a health issue would cut back a bit on the amount of curiosity people would have about your 4-5 years worth of "black hole", but it will probably vary by organization. I don't know the tech industry, but the only slight reservation I would have with ISayMoo's advice is to avoid having your resume read by an algo and your cover letter never ending up in the hands of a real person. This is where you can use your time over the next couple of months: try to leverage the contacts you have made on the research circuit and via previous projects for the purpose of getting your resume in front of hiring managers. The proverbial foot in the door... I would worry less about the perceived seniority of your first post-doctoral position and more about who you will be working for and with. That matters. 

I'm thinking ML that automates all the baggage carried by HR recruiters. Just as wrong, but faster!

It would be nonsensical to discriminate against a candidate because of a gap in their CV. Different things could have happened in your life. Besides, everybody knows that the most talented people have problems or drop out of the academia these days. Serious companies have their own recruitment systems.  If you perform well during the interview, you will score enough points to get the job. Show motivation, interest in the area, excitement about the role, ... - that's what they expect from you apart from the declared skills and knowledge. After all, they can kick you out after the probation period. And it's way more informative for the interviewers to read your research paper than some unverifiable claims in your CV about the scope of your work at some company.

You could even try to put a ('somewhat' don't overdo it) positive spin on it, by emphasizing you needed to gain some life experience/figure things out outside of academia to grow into the mentally stronger more mature person you are now.

Don't tell a thing on the mental health issues - even ignoring that, with your background you've almost zero chances of getting a job anywayz, mention "mental health" and you're certain to enter negative territory (not just not get a job but get blacklisted).

Anywayz, jobs are stressful and a PhD is piece of cake from what awaits you in the private sector. Not good at all if you had coping issues even with the easy option.

It would be nonsensical to discriminate against a candidate because of a gap in their CV. Different things could have happened in your life. Besides, everybody knows that the most talented people have problems or drop out of the academia these days. Serious companies have their own recruitment systems.  If you perform well during the interview, you will score enough points to get the job. Show motivation, interest in the area, excitement about the role, ... - that's what they expect from you apart from the declared skills and knowledge. After all, they can kick you out after the probation period. And it's way more informative for the interviewers to read your research paper than some unverifiable claims in your CV about the scope of your work at some company.

I somehow missed this post a couple of weeks ago, but I will say that almost every single statement made here conflicts directly with my experience. And I’ve spent most of the last quarter century as a hiring manager of PhD types, most of the time for what I think would be considered “serious companies”. You can’t have gaps in your resume. There is nothing wrong with “screwing around time”, but it needs to be documented as such. Otherwise, the default assumption is that you served the time in prison. As for those most talented people who drop out of school? Let’s say most of us are really not looking to hire the next Steve Jobs or Bill Gates. Very difficult people to manage! Finally, if you have ever fired somebody at this level (we are not talking strawberry pickers here, however expensive they may be), you would not casually talk about kicking people out after a probationary period. In the US we can, more or less, kick people out whenever, but it is almost always a painful process for all involved.

Fair enough. I was just limiting myself to situations that I’ve had first hand experience with - in this case being on either end of PhD level job terminations.

Bearish, that's why I think that someone like the OP should target research groups in tech companies like Google, Facebook, etc. Managers there are not supposed to "manage" them, but to guide them through the internal career process. Besides, I know people who sit years in the same company doing literally nothing - I'm sure they are easy to manage.

Bearish, that's why I think that someone like the OP should target research groups in tech companies like Google, Facebook, etc. Managers there are not supposed to "manage" them, but to guide them through the internal career process. Besides, I know people who sit years in the same company doing literally nothing - I'm sure they are easy to manage.

Good points. Although to the extent those companies turn into regulated utilities, in addition to being vast corporate organizations, the difference between them and large financial institutions may shrink considerably over time.