Mark Hamill first appears in "Portrait of the Artist as a Young Milo" when Pepper Ann suggests that Milo (who has artist's block) talk to someone whose work has inspired him in the past. Milo remembers that Mark Hamill has inspired him, and finds him at a book signing.
Unfortunately, the artist Milo was thinking of turns out to be Dorothy Hamill, the ice skater...not Mark.
Mark Hamill appears again in "Complimentary Colors" when someone votes for him as student-body president of Hazelnut Middle School via write-in...and he wins.
He appears once more in "The Finale" and has apparently become President of the United States.