Matt began DMing in his freshman year of high school after playing in a campaign with a particularly bad DM.
When Matt created the campaign that eventually became Critical Role, he made up his own world of Exandria and restructured some of the game system rules to allow for more flexibility in the storyline. He has been the Dungeon Master since the beginning of the campaign in 2012.
Prior to the Geek & Sundry streaming gameplay sessions, the group would meet every month to month-and-a-half for about eight hours. This infrequency was problematic, as many of the players needed to be reminded by Matt of elements of the storyline, how to play, etc.
When Matt told the other cast members about Geek & Sundry approaching him about making Critical Role, the group's reactions were recorded for posterity.
In preparation for the stream, Matt converted the game over from Pathfinder to Dungeons & Dragons, because Pathfinder combat is slower due to an abundance of various modifiers. D&D also has more name recognition. When converting their game, Matt had to create a custom fighter's Martial Archetype for Taliesin Jaffe's character, Percy, as the gunslinger doesn't exist in 5th Edition D&D. They both sat down and took a full day to switch over the character and all of his stats and abilities.
When asked if he could be any player character in the game, Matt has repeatedly chosen Scanlan.