The earliest published work by Ditko that i've been able to determine is actually a cover, oddly enough. Even back at his beginnings, they could see the value of his work. From October of 1952, the cover to Strange Fantasy #2 (not to be confused with the first issue, which was also labeled #2) -
Yes, he did interior work, too, and his art appeared regularly in the title over the next year.
At the end of that year, this odd little gem showed up. See if you can determine what made this Ditko tale from Black Magic #27 so strangely significant -
What made this tale stand out was the creature. This panel is the only one in which the creature is drawn by Steve Ditko:
The rest of them? Jack Kirby stepped in to redraw the creature in the other panels.
Man - teaming up with Jack Kirby in your first year working in comics? Not bad.
Lastly, for now - Ditko's first appearance in Space Adventures. He'd return to this book at the end of the decade to bring us Captain Atom before he launched into his own title. Back in 1954, it was science fiction, not superheroics, and emblazoned on the cover. And, again, that cover is by Ditko -
page art from Black Magic #27 (1953) and Space Adventures #10 (1954)