First appearance of his feature: Adventure Comics #48, cover-dated March 1940.
And that initial story was also reprinted in Justice League of America #96 (February 1972):
Rex was also a founding member of the Justice Society of America,
But, unfortunately, after All-Star Comics #7, Rex became the first member of the JSA to be removed from the line-up without graduating to his own solo-title (fellow Adventure Comics feature Starman took over for Rex in the JSA), and years later it was retroactively revealed that Rex had developed an addiction to "Miraclo", the pill he had invented that granted him one hour of superhuman abilities.