Jack O'Lantern was the main antagonist in the Halloween special, Billy & Mandy's Jacked Up Halloween.
According to Grim, Jack was a prankster in medieval Endsville, which is presumed to maybe be the 1500-1600s. Despite being pleasant, 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 too (walking teeth).
He had a problem with it, doing it constantly, apparently bad enough to force the townspeople to have an intervention about him, and ultimately, getting rid of him by sending their queen a prank gift and framing Jack for it. She sent a knight to his house to kill him.
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, the scythe for immortality, so he could keep playing pranks forever.
Unfortunately, Grim (who doesn't like being tricked) lopped off his head after giving him immortality, causing Jack to be shunned by society and forcing him to only be able to play his pranks on Halloween.
Before he managed to start his yearly pranks in Billy & Mandy's Jacked Up Halloween, he ran into Billy with Grim's scythe. He tricked Billy the same way he tricked Grim, stole the scythe, and then used it to possess pumpkins with the spirits of demons, with the ultimate goal of chopping off Grim's head with the scythe as revenge.
After Jack was defeated, Grim sent him to the underworld. There, he was beaten up by many humorless demons for pulling a prank on them.
Jack also makes an appearance in the video game.
Grim described past Jack as "a pleasant guy". After getting decapitated, Jack became bitter, and holed himself up in his house, only coming out on Halloween to play pranks.