Billy & Mandy in Trepanation of the Skull and You (1996 Billy & Mandy Short Film)

In 1996, while Maxwell Atoms was still in college, he created a short animated film titled Billy & Mandy (named after its two protagonists, a young boy and girl named Billy and Mandy, respectively). This short would go on to form the basis of Atoms' hit Cartoon Network show The Grim Adventures of Billy & Mandy. The character designs for the original short differ quite a bit from the end result, with Billy appearing taller and lankier than his final design, and with a smaller nose, and Mandy sporting pigtails, a nose (which she lacked in the series), and a black jumper with overalls.

The plot involves Billy wanting to release his inner demons, deciding to go about doing so by drilling a hole into his head. Atoms described it as being "completely inappropriate for Cartoon Network", though he felt that the characters were too fun to use only once, and decided to evolve the short into an entire series. In need of a straight man for the otherwise uncontrollable group of misfit characters, it was eventually decided to add the Grim Reaper to the lineup as the third protagonist.

In Atoms' 2013 RebelTaxi interview with Pan-Pizza (of thatfellowinthecoat.com and partner of Channel Frederator), he mentions the short in some detail, revealing that he has it stored away on film, even suggesting that he should have it digitally transferred "at some point".