According to Grim, Jack was a prankster in medieval Endsville, which is presumed to be between the 1500-1600s. He was pleasant, but he loved to pull pranks on the townspeople (sawing chair legs, changing road signs, "kick me" signs, etc). He is also shown to have (presumably) invented a couple pranks and gags too (such as chattery teeth). The problem with his pranking was that he never knew when to stop, sometimes pulling several pranks in a row on the same person, and occasionally pulling pranks that resulted in someone getting hurt. Jack was obsessed with inventing new tricks to pull on the people, staying up creating various prank devices, then "laughing himself to sleep" when he was finally done.

The townspeople grew sick of Jack's behavior, and all met together to "teach Jack a lesson once and for all". They accomplished this by sending their humorless queen a prank gift and framing Jack for it, knowing that The Queen would punish him. Furious, she immediately sent a knight to his house to execute Jack for the harmless prank.

When Grim came to reap his soul, the prankster refused to go. Jack managed to steal Grim's scythe (by tickling him into dropping it), and struck a deal with him; he would return the scythe in exchange for immortality, so he could keep playing pranks forever. Grim had no choice but to grant Jack his request.

Unfortunately for Jack, this enraged Grim (who doesn't like being tricked) and so Grim sliced off Jack's head after giving him immortality, resulting in Jack needing to replace it with a pumpkin, causing Jack to be shunned by society and forcing him to only be able to play his pranks on Halloween, when he could fit in with the ghouls and costumed people. This slowly caused him to become bitter towards Endsville and society, but most of all, towards Grim.