While Martian Manhunter's post-Rebirth status quo is somewhat up in the air, the fan-favorite character has apparently been restored to his role as a founding member of the Justice League, thanks to a flashback sequence in this week's Justice League #37 by Christopher Priest and Philippe Briones.
The single-panel appearance by J'Onn J'Onzz suggests that the Martian Manhunter was indeed a part of the earliest days of the Justice League -- a status that he had enjoyed since...well, since it happened in the actual comics.
Martian Manhunter joined Wonder Woman, Green Lantern, Aquaman, and The Flash to found the Justice League in The Brave and the Bold #28, the team's first tale. Superman, Batman, and Wonder Woman have been retroactively removed and replaced a few times as founding members, but Martian Manhunter had always been one of the founders -- right up until 2011's The New 52 reboot, in which J'Onn's role on the team was seemingly swapped out for Cyborg, a longtime member of he Teen Titans.
As in the Justice League movie, Cyborg's admission into the League played a major part in saving the day during their first post-Flashpoint adventure.
When Brad Meltzer elected to leave Martian Manhunter out of the Justice League lineup for his 2006 reboot, it was the first new iteration since the team was founded in 1960 in which J'Onn was not a member. (There was a brief period in the '90s when Martian Manhunter left the team, but it was later revealed that a character known as Bloodwynd had secretly been a shape-shifted version of J'Onn all along.)
While Martian Manhunter has been absent since Rebirth, his "classic" costume, with briefs and a collared cape, appeared in the Justice League's trophy room alongside dozens of other Easter eggs in The Flash #21 last year.
Whether J'Onn's presence will be addressed in-story or not has yet to be revealed: "The People vs. The Justice League" deals with a Justice League superfan who is trying to "help" them by killing villains, but the effect is that he is turning everyday people against the League.
You can get a copy of Justice League #37 at your local comic shop or on ComiXology.