Was Superman ever a Green Lantern?

Answer 1

Technically and officially, the answer is "no." There was an Elseworlds special Superman: Last Son of Earth, released in 2000. This tells the story of Superman (Clark Kent) growing up on a doomed Earth, from which he is sent into space and arrives safely on the planet Krypton which never explodes; a reversal of the legendary origin. Adopted by Jor-El, Clark grows up to find a discarded Green Lantern power ring and uses it to represent his adopted and native planets. A sequel was printed in 2003, titled Superman: Last Stand on Krypton. Neither of these stories occurred in current continuity and is considered an alternate history tale.

In an alternate future, depicted in Action Comics Annual#3 ("Executive Action," 2001), Superman gets an opportunity to become a Green Lantern when he defeats a rogue Guy Gardener and, taking possession of his ring, is then offered a lantern/power battery from Hal Jordan and John Stewart. He passes on the opportunity, and it is just as well, as that future was merely a potential future outside of canon anyhow.

One appearance of Superman as a Green Lantern that does occur in canon is Superman #147 ("One Man JLA: From Krypton to Oa," 1999 (the four-part story continues in Adventures of Superman #570, Action Comics #757, and Superman: Man of Steel #92)), however this turns out to not be occurring in reality and so this never truly happened either. Superman was also portrayed as a Green Lantern when he fell victim to a similar scheme orchestrated by the villain The Key in JLA #8-9 ("Imaginary Stories," "Elseworlds," 1997).