His most notable role was as the FBI's Assistant Director Walter Skinner on "The X-Files". The role was originally recurring, but the part expanded, and in 2001 he became a regular member of the cast. The character gradually became the most integral character besides the two leads. He remained with the show until its end in 2002. Pileggi played the character in the 1998 "The X-Files" film and in the 2008 film "". He also reprised the role in the 2016 miniseries.