Late in the 19th century a man named Cyrus Gold was murdered and his body was dumped in the waters of the Slaughter Swamp, located near Gotham City. While rumors of sacrificial rituals being performed in the swamp by early Gothamites persist to this day, it wasn't until a nearby slaughterhouse started dumping offal into its waters that it earned the name 'Slaughter Swamp'. This history of ritual and death led to the swamp becoming a haven for the malicious elemental force called, 'The Rot'. The Rot feeds off of death and it reanimated Gold 100 years after his murder to become one of its avatars.

The shambling creature that lurched from the swamp bore almost no resemblance to Cyrus Gold nor did he possess his mind or memories. The only thing he was able to speak were lines from the 19th century English nursery rhyme, 'Solomon Grundy,' which is how he earned the name he now goes by. Since his reanimation, Grundy has been killed a number of times, only to rise again later from Slaughter Swamp. Interestingly, over successive reincarnations, Grundy has displayed varying personalities. Sometimes he has returned as a mindless brute; at other times he returned with some of the memories and intellect of Cyrus Gold and has been able to resist the Rot's directives to a degree.