As I'm sure most of you are aware, Julius Schwartz was the editor for the revived All-American Comics during the Silver Age. The Flash, Green Lantern, the Atom, and Hawkman all returned under his watchful eye, as did the Justice Society (rechristened, of course, as the Justice League) of America. Schwartz edited all those magazines from their Silver Age debuts (plus the New Look Batman and Detective) until late 1967. However, when the Spectre got his own (brief) series, Schwartz was relieved of responsibility for Hawkman effective with issue #22 (Oct-Nov 1967). The new editor was George Kashdan.
As was usual back then, a restuffing of the editorial chair also meant an entirely new direction for the series, including new artists (Dick Dillin replacing Murphy Anderson), and a new writer (Bob Haney filling in for Gardner Fox). In the opening story, it is revealed that Carter Hall is actually an alien from Thanagar:
Say it loud, say it proud, Carter. At first he is arrested, but when Hawkman defeats the villain of the issue, a grateful city hall has him released. What's that? You want to know how Hawkman could beat a villain while his real identity was in prison? Well, actually Carter was a Thanagarian android that Hawkman had sub for him.
Now, you might expect there to be one of those complicated excuses where it is then established to the public's knowledge that Carter was in fact an Earthman, and was only pretending to be an alien to further some goal. But no:
And you might expect lots of interesting plot complications in future issues as Carter had to deal with reactions to his alien nature. No to that as well. In fact, as far as I can tell it was never mentioned again, except for this note in the letters column of Hawkman #24:
By #26, Kashdan was out as editor, and Murray Boltinoff was in. Hawkman's solo series was canceled after #27, although he was combined with the Atom to form the Atom and Hawkman series for another year, where he was reunited with editor Julius Schwartz. However, I can find no other mention of Carter Hall being an alien.