Great question! I think what you “need” to be an editor depends on a lot of things, like which niche you’re targeting, where your clients are, how you’re getting them, etc. It also depends on what you have to “show” for your writing skills in the event that a gatekeeper asks for “proof.” To that end, special distinctions or accomplishments (or even samples of your own work) can matter more than actual degrees. And, of course, the best qualification is always experience. But in a word, no; plenty of successful writers (and editors!) don’t have an MFA, so I don’t think that’s a prerequisite.
Now I’ll try to be a bit more detailed about how to leverage what your background is (buckle in!):
Lots of people mistakenly believe that editing is simple proofreading. It’s not, but if it were, then impeccable grammar/punctuation/spelling would be enough, and some people master those by high school… or, you know, nowadays we could all just use Grammarly. ;) It’s beyond the proofreading component where having “niche” qualifications begins to matter.
So ideally you’d be targeting some kind of special niche, in which case it becomes a question of whether your background experience has “schooled” you (formally or otherwise) in that field’s conventions: the structure, word choice, tone, psychological impact (and techniques to use to create that impact), and so on. For example, maybe your proofreading skills are flawless—okay, cool — but if you’re editing a business newsletter when you know nothing about marketing, you might be totally clueless about any of the more nuanced problems in word choice, structure, and so on… and not even realize that you were letting those problems slide.
Or if you were editing a document targeted toward an audience with whose culture you weren’t personally familiar, then you might totally miss the fact that a given phrase or anecdote within the text represented a taboo that was going to turn the audience off. That’s yet another situation where experience matters more than written language or the simple having of a degree.
Personally, for instance, I don’t have an MFA, but I have an M.A.+ in “researchy” stuff — and I’m primarily an academic editor. Stopping at a B.A. wouldn’t have been adequate for me to edit serious ivory tower research, nor would an MFA have taught me anything about how to do that, because unless you’ve ever been directly involved in the writing of a serious research publication, with a mentor, you don’t truly learn about the structure/language of serious, professional-level research writing. To some extent, I can edit stuff I don’t have direct experience with, but I tell my clients if they send me a project that represents something “new” for me; for example, job talk papers, personal statements in fields outside of the social sciences and humanities, revise-and-resubmit letters for academic journals, and so on. (I’ve edited all these things without issue once I had templates to study, but I still feel it’s important to be honest with clients when you have limited experience with what they’re requesting.)
Meanwhile, concrete examples of language-/writing-oriented work and coursework you have already done are valuable, even if they don’t add up to a degree. For example, if you want to do content editing, it helps if you have creative writing credits, a stint in teaching literature or composition, or some other sort of “hard proof” that you have a solid grasp of that deeper side of story-building. But those details can look very different from person to person.
Back to my example: my B.A. was literature-focused, I taught composition for a year, and I took a some creative writing electives with faculty from a very highly-ranked MFA program. All these experiences add up to a sound grasp on “effective” storytelling. There’s always more to learn — but it’s still a good amount of preparation.
On the other hand, one of my friends never finished college but has occasionally done editing gigs too — she’s super-smart, reads a ton, and has impeccable written English. In her case, not having a degree didn’t seem to be a major obstacle; word of mouth still spread, and she got her clients. Granted, that has never been her primary job, but she’s proof that it’s not utterly impossible to get this kind of work without a degree.
So all of the above is pretty much a long way to say that it’s a matter of determining what you genuinely believe you are qualified to do, and then marketing yourself appropriately. ;)
Above, I also referred to “where your clients are, how you’re getting them.” Here’s what I’m talking about:
Some channels for finding clients are more stringent with respect to “qualifications” than others. For example, on the one end, you have online freelancer databases where tons of unqualified people are competing for editing jobs. Maybe their English is shaky, their written English is even worse, but they continue getting jobs because their clients don’t know the difference and because they’re charging WAY under the average rates because they can afford to (e.g., they’re from countries with low costs of living). I don’t (at all!) assume you’re unqualified, but I mention this because it’s a good example of how, on some platforms, the things you might perceive as “gaps” in your qualifications can be easily overlooked.
On the other end with respect to “platforms,” you have stuff like university editor lists. These might be easy to get onto, and as long as you’re genuinely producing high-quality work for the students and researchers who come to you, you’re likely to be able to remain on them… but if your work were shoddy, the universities would eventually figure it out and you’d be off. Alternatively, some universities won’t even recommend your services unless you have a Ph.D. or have worked for an academic publisher. It’s all different.
So I guess in summary, it all depends on where you’re targeting your services and whether you have a defensible way to present yourself and your preparation. Price matters too, and it should reflect how much you truly believe your services are worth. I struggle with this one because I know I could charge more (way more) than I do for my own niche and experience level, but I try to keep it affordable for grad students. At the beginning, though, I charged even less — because while I was already qualified, I was less experienced.
Finally: having a website helps immensely. In an economy where everybody thinks, “Hey, proofreading is easy, I can try to make a couple bucks!” it reeeeally helps to show that you are serious about the job — and it provides a good showcase of your own writing skill — if you have created a business website. (Example here.)
I hope that wasn’t information overload, and I hope that helps!