In September 1998, while in an official meeting of OPR with Special Agent Fox Mulder, Arnold expressed his opinion of Mulder's extreme arrogance, considered his recent report "unintellible," and informed him of his reassignment off of the X-Files. (TXF: "The Beginning")
In 1999, Arnold delivered a report on federal crime projections for the next year. His report was interrupted by Arlene when Special Agent Mulder contacted them about an impending bank robbery. (TXF: "Monday")
- His report was interrupted several times, with the day resetting each time until Pam's death.
In 2000, Arnold met with Special Agent Gene Crane, Deputy Director Alvin Kersh, Assistant Director Walter Skinner, and Special Agent John Doggett to discuss the recent cult slayings by Anthony Tipet. Arnold was skeptical of Skinner's insistence that there was some supernatural activity involved with the slayings. (TXF: "Via Negativa")
AD Arnold was portrayed by Wayne Alexander.
In "Monday", Mulder refers to the character as "Agent Arnold". This does not contradict the fact that he was shown to be an Assistant Director earlier that year, in "The Beginning", as several other ADs in The X-Files - including AD Skinner - were also each referred to as an Agent, establishing that the two ranks were either equal or that the Agent rank included that of Assistant Director. It is also possible, however, that Arnold was no longer an AD at this time.