Which athlete was the first doping case in Olympic track and field?

That was a Pentathlete, Hans-Gunnar Liljenwall, in Mexico City, in 1968. That case actually involved alcohol, not a performance-enhancing drug.

I consider the Pentathlon to be more or less a track-and-field event, but some people might not. If not, then Ben Johnson was really the first important track and field athlete to be disqualified for doping.

See the history, below:

https://en.wikipedia.org/wiki/Doping_at_the_Olympic_Games