Was depression nonexistent in Ancient to Late Medieval Times?

It's impossible to diagnose someone from the past. And we know little enough about the minds of common folk, in general.

I think the description of people as having no time to think on such things is misleading. The nights could be long and dull plenty of time for contemplation if one was so inclined. For example, this was a very religious time and we know people where very worried for their souls and the souls of loved ones (exploiting this in the sale of indulgences is part of what lead to Luther's contempt for the Papacy.) These are fairly esoteric concerns.

It is dangerous to label anyone with a mental illness. There are examples of Medieval people who some have speculated were depressed but it's an irresponsible path to go down. However, more broadly, descriptions of melancholia or sloth from the Middle Ages do resemble our understanding of depression.

Here is John Cassian's writing about "acedia" a sort of affliction that somewhat resembles depression. It was known to impact monks and those who lead solitary lives. Our current day understanding of depression tells us that being solitary can lead to depression.

Cassian was French monk (wouldn't have been called France yet) who died in about 435.

http://www.osb.org/lectio/cassian/inst/inst10.html

OUR sixth combat is with what the Greeks call akhdia, which we may term weariness or distress of heart. This is akin to dejection, and is especially trying to solitaries,

...

AND when this has taken possession of some unhappy soul, it produces dislike of the place, disgust with the cell, and disdain and contempt of the brethren who dwell with him or at a little distance, as if they were careless or unspiritual. It also makes the man lazy and sluggish about all manner of work which has to be done within the enclosure of his dormitory

Then the fifth or sixth hour brings him such bodily weariness and longing for food that he seems to himself worn out and wearied as if with a long journey, or some very heavy work, or as if he had put off taking food during a fast of two or three days. Then besides this he looks about anxiously this way and that, and sighs that none of the brethren come to see him, and often goes in and out of his cell, and frequently gazes up at the sun, as if it was too slow in setting, and so a kind of unreasonable confusion of mind takes possession of him like some foul darkness and makes him idle and useless for every spiritual work, so that he imagines that no cure for so terrible an attack can be found in anything except visiting some one of the brethren, or in the solace of sleep alone.

Another common feature of depression is thinking everything would be better if XYZ (if you were somewhere else, if you started showing up to work early and getting everything done, etc.)

Then the disease suggests that he ought to show courteous and friendly hospitalities to the brethren, and pay visits to the sick, whether near at hand or far off. He talks too about some dutiful and religious offices; that those kinsfolk ought to be inquired after, and that he ought to go and see them oftener; that it would be a real work of piety to go more frequently to visit that religious woman, devoted to the service of God, who is deprived of all support of kindred; and that it would be a most excellent thing to get what is needful for her who is neglected and despised by her own kinsfolk; and that he ought piously to devote his time to these things instead of staying uselessly and with no profit in his cell.

I want to be clear that I'm not necessarily saying we can conflate the two. Because it's irresponsible to diagnose those in the past. But I do think it goes to your broader point. Feeling dejected, unable to work, exhausted etc. all things we associate with depression was not unknown. Cassain writes about having read about this elsewhere.

In Chaucer's the Pardoner's Tale there is the description of wanhope (lack of hope). Which, according to the Pardoner, is having such despair and feeling so very sinful and down about yourself that it will lead you to commit unforgivable sins, such as suicide.

Now comes wanhope, that is despair of the mercy of God, that comes sometimes of too much outrageous sorrow, and sometimes of too much dread, imagining that he has done so much sin that it will not avail him, though he would repent himself and abandon, through which despair or dread he abandons all his heart to every sort of sin, as says Saint Augustine.

And further on:

his horrible sin is so perilous that he that is in despair, there is no felony nor no sin that he fears to do, as showed well by Judas.

By that he is referring to suicide, which is Judas's fate according to Matthew.

Again, this isn't necessarily describing depression. But it does suggest Medieval people were familiar with more broad concepts of deep despair, suicidal thoughts, loss of hope. They weren't so preoccupied that the general concept would be alien to them.

Monks had a fairly privileged life and Chaucer was mostly writing for the elite (although the Canterbury Tales certainly had a lot of cross appeal). But most of the work we have from this time period are written by or for the privileged. I wouldn't assume these concepts would be foreign to the common folk.