According to Who's Who: The Definitive Directory of the DC Universe #16, the only known fact regarding the Pre-Crisis Mongul's early life is that he was once the ruthless overlord of a world with billions of subjects, who was eventually forced to flee after a revolution led by a holy man known as the Arkymandyte.
The only known fact about Mongul's early life is that he was once the overlord of an entire world, a tyrant who ruled mercilessly over his billions of subjects. One day an ancient holy man, the arkymandyte, came from out of the hills to inflame Mongul's people in protest, and finally, to revolt against his evil reign. Mongul was forced to flee his world to save himself from death at the hands of enraged mobs.
Who's Who: The Definitive Directory of the DC Universe #16 (June, 1986)
These details were originally divulged by Mongul himself in DC Comics Presents Vol. 1 #28.
MONGUL: For years, I was overlord of an entire world, absolute ruler over billions of beings--and thus, my narrow-minded subjects branded me a tyrant! They did not understand that, to rule well, one must rule with an iron hand... they simply did not understand power! Perhaps that is why they turned against me in the end--and turned to the ancient Arkymandyte who came shuffling down out of the mountains... he had been hiding there for decades, waiting--and now his time had come! The holy man knew power, too, it seemed--the power of the word--the power of religious fervor! In a matter of days, he had stirred my people to the boiling point--and, at last, I was forced to flee the throne for my life! There was, of course, one small satisfaction... the Arkymandyte promptly became an even greater tyrant than I'd ever been!
DC Comics Presents Vol. 1 #28 (December, 1980)
There aren't any comics which talk about Pre-Crisis Mongul's origin in any more depth than this.
In regard to Mongul's strength level, relative to Superman's, the Who's Who says the following:
Mongul's abilities appear to be almost on par with those of Superman, witnessed by the fact that he was repeatedly able to stand in battle with the Man of Steel. Mongul can issue forth blasts powerful enough to force Superman to a standstill. In addition, Mongul can teleport as well as produce dimensional-inversion cubes that can warp their interior reality.
That's as in-depth an explanation of Mongul's powers as was ever given within an official DC Comics publication. DC tends not to explain or quantify the powers of their characters as precisely as Marvel often does with theirs.
As far as head-to-head showings go, the final and most definitive battle between the Pre-Crisis versions of Mongul and Superman occurred in Superman Annual Vol. 1 #11. After knocking one another back & forth for a while, Superman appeared to have gained the upper hand, and Mongul said he almost believed that Superman was going to kill him. However, Superman hesitated, and Mongul used that opening to strike back and gain the advantage himself. Then Robin intervened, using an alien parasite to render Mongul catatonic, thus bringing the battle to an end.
Superman Annual Vol. 1 #11 (September, 1985)
All in all, it seemed like Mongul and Superman were roughly equal in strength. You could make a case for either one being slightly stronger, but the evidence is very much open to interpretation.