We actually have very little published information on Mordenkainen, and his characterization has changed focus quite a bit over time. Unlike Elminster, who is the subject of a series of novels, Mordenkainen is only given passing mention in the sourcebooks, and he's really not in that many of them.
In short, we are told that he does things, but--as far as I can find--never given actual record of what he's done. Though, as you can see below, the development of his character has rather expanded throughout the history of D&D.
In D&D 1st Edition, Mordenkainen appears in the module Mordenkainen's Fantastic Adventure, where we get nothing useful about his personality--just a description of what he looks like, and that he's very well prepared for the job.
In Second Edition, he appears in the City of Greyhawk Boxed set, where this is said about himself and his companions...
Their [the circle of 8] actions are usually directed toward preventing any country, faction, or organized group from growing too powerful and overwhelming others, spreading a growing influence across the lands of Oerth. The history of this young world has taught them that great dominance arising from any quarter leads to great wars, hubris which may draw down the wrath of one or more deities, or even greater disasters
-Greyhawk: Folk, Feuds, and Factions, p.21
A description of his personality is in the Epic Level Handbook
Currently, Mordenkainen leads a powerful group of wizards known as the Circle of Eight, who work to ensure that no one power gains dominance over too much of the Flanaess, and that a balance of power remains constant throughout Eastern Oerik.
Mordenkainen sees himself as a chief instrument of the Great Balance. His philosophies are more complex than simple equality—they are highly detailed and extremely theoretical ideals derived from decades of arcane research. Mordenkainen has fought ardently for the forces of good (most recently during the Greyhawk Wars) with his private army, the Obsidian Citadel,but just as often he has worked the side of infernal forces. [...] As a result, the wizard is not trusted, even among the likewise neutral druidic Hierophants of the Cabal, who find his vision of balance wholly self-centered and some-what arbitrary.
This time, we get a description of his beliefs straight from his 'own mouth' in Mordenkainen's Magnificent Emporium
Once I sought power for its own sake. Now I seek it to keep it from others. I do not wish to rule the world—only my small part of it. Yet many are the grasping hands of those who would claim control. Some use evil means toward such ends, while others pursue a more virtuous path, but regardless of the manner of its acquisition, power wielded to dictate over all is power used to disastrous effect. Friends will be lost, great works will be cast to ruin, and wars will be fought, but none shall gain the upper hand over all as long as I and the Circle of Eight remain.
We balance the scales.
Interestingly (and in contradiction 5th edition), it also says this:
I care little for [other worlds], as I am from this world, this Oerth. Just as these other worlds have similar magic items, I’m sure there are some among them who serve as I and the Circle of Eight do. Let them sort out their own problems; we have enough of our own.
He shows up a few times in 5E, but the most succinct description we have of his belief structure is found here. To quote a few key lines...
they seek to maintain equilibrium across the cosmos above all. Mordenkainen and his compatriots are among its most notable devotees.
To an outside observer, a disciple of the Balance might act cruelly or heartlessly one day, and benevolent and caring the next. A follower might aid in consigning one city to domination by a devil cult while driving demonic cultists from another. For the sake of the Balance, the cosmos must remain in a static state where neither demon nor devil can permanently gain the upper hand.
In conclusion: Mordenkainen started off as an individual who believed that if any mortal organization grew too powerful, it would bring more harm than good. Keep things small, and bad things happen on a small scale. It's only been over time that this has expanded to his current "I'm the Blood War's referee" state.
But all of this is told to us, not shown. We don't get details beyond a very high level.