If you don't get an answer here, you can also try /r/askhistorians, /r/academicpsychology, /r/historyofscience, /r/historyofideas, or perhaps /r/philosophyofscience

Layman comment: but I feel like there's references in medieval Europe to kings falling to deep malaise / melancholy.

Also would be interesting to look at early buddist philosophers and ask how they were so so early on the drop to realizing nothing is real in the way we think of it, and what biochemically may have made that a widely swallowable truth

But hey I'm just a know nothing commentor so... Ask others!

I've learned it's on mesopotamian clay tablets, with a description of symptoms and treatments - although it was handled by priests, since they thought it was caused by demonic possession or curses (by either a god or a human). Does trying to find out which cause "applied" in each case as early psychotherapy?

The eighth and ninth clay tablets of the Gilgamesh epos describe his grief and wanderings after Enkidu's death, with an account we would read as depression today.

Later on Herodot and Plutarch describe it as a disease in their medical texts, but they called it melancholia.

It was reported among the ancient Greeks, as was a treatment they sometimes used, throwing the patient off a short cliff into the sea. (I read about this many years ago when I wrote an educational monograph on depression for Bristol-Myers Squibb when they were launching the antidepressant nefazodone.) Presumably it existed before this, but I don't know when it was first recognized as a disease. FYI, Hippocrates (ca. 400 BC) thought that all illnesses resulted from an imbalance in the four humors: blood, yellow bile, black bile, and phlegm. If you substitute these with the neurotransmitters serotonin, norepinephrine, and dopamine, he may not have been that far off from what we know now (which isn't much).

Here's a lay article that summarizes historical findings.

Tl;dr: a long time ago - I think someone else noted that in the Epic of Gilgamesh, he goes through some significant bereavement after his friend's death.

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The podcast Sawbones (maximum fun network) is on medical history, and I’m sure there are a more than a few episodes on depression. The host always goes over (ancient) historic records, so you may get some specifics there as well.

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Interesting response. Thank you.