He was introduced to comic books in 1961 in Superboy #81 as “Superboy’s Big Brother”! Visually, his costume was the reverse of Superboy and since he possessed the same powers as Superboy, he was thought to be from Krypton as well. Therefore, the name, “Mon-El” – he arrived on Earth on a Monday and El is Superboy’s family name! At the end of the story, Superboy indadvertedly poisons Mon-El with lead but ‘saves’ him by sending him into the Phantom Zone (where Krypton imprisoned their worst criminals) where he remained till freed by the Legion of Super-Heroes in the 30th century.
Mon-El would join the LSH and become a key member, marrying Shadow Lass in the process. He was my favourite Legionnaire and he always seemed to me, a better character than Superman. But sadly, due to DC’s decision to start fucking around with their continuity (which is still going on to this day) in the mid-80s with Crisis on Infinite Earths, Mon-El’s character (and the LSH overall, to be fair) was altered irreversibly and his origins were ret-conned (damn I hate that word) and he was renamed “Valor” (fucking awful!) and then later “M’onel” (even worse!).
The character has been ret-conned so many times now – he even became a Green Lantern at one point in time (!) – and this is one of the many reasons why I have stopped reading superhero comic books. That said, I will always remember the Mon-El I once loved – and that is something Detective Comics Comics cannot take away!!