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For some reason, the writers and editors chose to ignore J'Onn's non-Supermanque powers like telepathy, invisibility, and shape-shifting, and just had him using the standard flight, strength, invulnerability, and Martian Breath by the early 60s. This, I think, contributed heavily to the demise of his own feature and his leaving the Justice League. If he was nothing but an imitation Superman, why was he necessary?


As you implied, writer Gardner Fox and editor Julius Schwartz are almost certainly to blame for the mishandling of the Martian Manhunter, and the massive under-utilization of his varied and unique powers (I rather suspect they basically did little to no research, or reading of back issues of Detective Comics), as more than one reader bemoaned in the letters' page of 1960s issues of the JLA.

If there is one major drawback about the JLA under Gardner Fox (in my opinion) it is that the Martian Manhunter really just comes across as a cipher or non-entity, and a weaker Superman-clone, to boot.