Plastic Man was originally a character published by Quality Comics (from 1941 to 1956, though the last few years may have been mostly been reprinted stories?), then it looks like in 1963, I.W. Publishing published three issues of a Plastic Man comic book, but those may have contained all reprinted stories originally from Quality Comics.
Plastic Man had his first DC Comics series back in 1966.
Well, he appeared in one panel of Justice League of America #107 (September-October 1973), the issue that first introduced the team, but that seemed to indicate the Earth-X version of Plastic Man had previously died in the fight against the Nazis.