Kant was the one who introduced the distinction between noumena and phenomena, but if we make abstraction of this terminology and its very specific meaning, I think we can trace this back to the very beginning of Western philosophy. In fact, the distinction between fundamental reality and our perception is very common in ontology.
Please note that my "overview" is far from complete. It is only meant to give some insight into thinkers before Kant who worked on similar ideas.
The Milesians were the first who were looking for the true nature of reality; the fundamental building block of everything. For Thales, this was water, for Anaximander the Apeiron and for Anaximenes air.
Please note that this is still very different from Kant's Critical Idealism. The Milesians didn't say that reality is constructed by the mind (even though Kant's Critical Idealism recognises the existence of a real, objective world, which is necessary for us to see those representations); they only said that there is a fundamental, underlying principle of reality.
Parmenides said that fundamental reality is unchanging and that the change we perceive is nothing but our senses deceiving us. There is the unchanging world of pure, real, reasonable knowledge, the kind of knowledge only the philosopher, which stands in contrast to the changing world of the imaginary and the deceiving, the kind of knowledge the layman has access to.
Again, this is still quite different from Kant's Critical Idealism, but it's getting closer in the sense that the real, objective, unchanging world (which can be compared to Kant's noumena) stands in contrast with the deceptive world of the senses (which can be compared to Kant's phenomena)
Parmenides will have a huge influence on Plato, which will show up in his Ideas and the Allegory of the Cave.
Plato's Allegory of the Cave distinguishes fundamental reality very clearly from representations.
As you can see in the picture above, the philosopher is outside the cave, driven by eros to find real knowledge (compare with Kant's noumena), while the layman is a prisoner in the cave, thinking that the images he sees on the wall (compare with Kant's phenomena) are real, while they are in fact nothing more than imitations of reality onto a wall.
This is why Plato is said to dislike art so much. Consider the Idea chair. When a carpenter creates a chair, he imitates the Idea chair. However, when an artist paints that chair, he is making an imitation (his painting) of the imitation (the physical chair, made by the carpenter); he is, in other words, getting further and further away from the truth.
Note that Plato is also not an idealist; he doesn't say that the mind makes up fundamental reality. On the contrary, his world of Ideas is the world of the unchanging, of being (i.e. independent of minds).