April 1958: Adventure Comics #258. Otto Binder and Al Plastino create the Legion of Super-Heroes, a club of teenage superheroes a thousand years in the future, who bring Superboy to their time and tell him that he was their inspiration. The group catches on, appearing a handful of times over the next few years.
September 1962: Adventure Comics #300. Superboy’s team-ups with the Legion become a regular feature in Adventure, where they will remain through May 1969’s #380. This first story is written by Superman co-creator Jerry Siegel and drawn by John Forte with Al Plastino.
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July 1969: Action Comics #378. The Legion becomes a backup feature in Action with this issue’s Jim Shooter/Win Mortimer story (following a reprint in the previous issue), and remains there for a bit more than a year. (Shooter, at this point, has been writing Legion stories since 1966–he sold his first script for the series at the age of 14.) Following its Action run, the Legion becomes an intermittent backup feature in Superboy for a couple of years, beginning with March 1971’s #172.
February 1973: Legion of Super-Heroes #1. The first series to be named after the Legion actually reprints stories from the mid-’60s. It lasts four issues.