PMO began in February 2006 at http://pmo.hostiz.com. A year later Hostiz was destroyed, so Douglas Buel of The Magic Lampoon gave PMO sanctuary at http://pmo.magiclampoon.com in February 2007. Finally it found a permanent home at the Magic humor site Good Gamery in April 2008.
The original intent of Pro MTG Online was to prove that a web comic better than any existing Magic-related web comic could be generated using only MSPaint and complex procedural algorithms. As competing Magic-related web comics perished around it, PMO continued onward to become the oldest living Magic web comic.
There are currently over 200 PMO comics.
At the 100 comic mark, a contest was held asking readers to submit their own PMO-style web comics. The winning submissions were published.
PMO is drawn using MSPaint, usually only in black and white. Its font is generally Arial 8pt, bolded. At first, special effort was put forth in order to make it seem like the artist cared very little about the visual quality, but this has changed somewhat over time (though not completely).
The 135th comic began a labyrinth which ended at a secret PMO forum, which was annihilated when Hostiz was destroyed. Now it leads... elsewhere.
There's secret "invisitext" on every comic.
It was originally claimed that the comic had an anonymous author, until it was eventually revealed that the comic drew itself.
In PMO #217, there is a 1/4 chance upon refresh that the comic will display one of three alternate texts.
In PMO #249, the actual content of the comic is in quantum superposition until observed.
Mark Rosewater's approval
In an October 19, 2009 interview with Mark Rosewater on PureMTGO.com, Mark Rosewater was asked about whether he read U/G Madness. His immediate response was instead about Pro MTG Online, and how he is poked-fun-at there.