Lar Gand of Daxam was a 20th century explorer. He landed on Krypton but was immediately warned by Jor-El of the planet's imminent destruction and was given a map to Earth. He then went into suspended animation. After a few years, he landed on Earth and met Superboy. Although Gand suffered from amnesia upon landing on Earth, he and Superboy became good friends. Under Earth's yellow sun, he had powers like those of Superman; this led Superboy to believe that Gand might be his previously unknown long-lost brother. Superboy named the amnesiac alien 'Mon-El': 'Mon' because he landed on Earth on a Monday, and 'El' for Superboy's Kryptonian family name.
The truth was soon revealed, but unfortunately not before Mon-El became fatally poisoned with lead, which is deadly to Daxamites. To save his life, Superboy projected Gand into the Phantom Zone, where he remained for 1,000 years among criminals, until he was finally released thanks to Brainiac 5, and he joined the Legion of Super-Heroes, of which his 'brother' Superboy was also a member.
Mon-El developed a long-term relationship with Shadow Lass, and eventually they married. However, not long after, he died of injuries inflicted by the Time Trapper.
Note:In the Glorithverse, Earth-Zero and New 52 universes, all of which appear to have succeeded the 'Pre-Zero Hour' period, Mon-El apparently survived his injuries.