Before the series
There is a painting of him in room 113B, which appears similar to a janitor's closet (which also leads to a brain washing room in the tunnels), further indicating that he was a janitor. His painting appears in The Theme Team and again (albeit briefly) in The Dance Part 1. In The Curse of Earl Nigma it is revealed that Earl Nigma was a gifted fencer but stumbled at one point during a tournament and ever since the A Nigma fencing team has been cursed to lose. Since Chaz is the source of this information its accuracy is highly suspect.
Earl Nigma was also an inventor. He and Finnwich were friends and it was Earl Nigma who made the viewing device for the hieroglyphs. Finnwich laments in Follow that Finnwich that he hadn't "believed him sooner".