First appearance: Flash Comics #1 (January 1940):
Johnny was one of several recurring features (which included The Flash, Hawkman, The Whip, and Cliff Cornwall) to premiere in that inaugural issue.
Johnny was also present at the first meeting of the Justice Society of America in All-Star Comics #3 (Winter 1940).
What, you don't see him seated at the meeting table on the cover?!?
Well, there's a reason for that:
Johnny wasn't technically an official member of that group, so he didn't get an invitation to the meeting.
(His actual membership would happen a couple of issues later.)