The Grim Adventures of Billy & Mandy is an American animated television series created by Maxwell Atoms , that originally aired on Cartoon Network , the show aired on June 13 , 2003 and ended on October 12 , 2008 with the finale Underfist . The Grim Adventures of Billy & Mandy is an American animated television series that originally aired on Cartoon Network . Having originally aired on Grim & Evil in 2001 , the show aired ( Under the The Grim Adventures of Billy and Mandy title ) on June 13 , 2003 and ended on October 12 , 2008 with the finale Underfist .