Cyrus Gold, a gangster in the late 19th century, was killed and thrown in the Slaughter Swamp in Gotham. Chemicals that have been poured into the swamp have brought him back to life, more as a decomposed swamp monster than a human.

He has lost almost all memory of his past life and finds a homeless shelter to live for a while. When asked about his past life all he can remember is that he was born on a Monday so he was called Solomon Grundy after the old rhyme which goes:

*Solomon Grundy,

Born on a Monday,

Christened on Tuesday,

Married on Wednesday,

Took ill on Thursday,

Grew worse on Friday,

Died on Saturday,

Buried on Sunday.

That was the end of

Solomon Grundy.*

Being zombie and lacking a soul, Solomon Grundy truns into a huge, extremely unintelligent villain who often works for others because he can't think of his own plans.

He can't be killed because he is already dead, so he was strapped to the moon but eventually came back.

There are many versions of Solomon Grundy. He is sometimes intelligent, sometimes kind but still a villain, but usually he is just big and ridiculously stupid.

It is important to know that other then the "Tornado Run" Solomon Grundy has never been shown as smart and intelligent. For reasons not explained other then he was sick of dying, Solomon became a gangster. Red Tornado killed him. Though Solomon Grundy can come back he also has feelings. He used to have a crush on Jade until Skyman died.

DC Animated Universe

The DC Animated Universe has a different story line than other comics. The comics are based on the TV shows and are part of the same universe. In the Justice League and Justice League Unlimited cartoons Solomon is his typical large and moronic self. He joins many supervillain groups lead by both Lex Luther and others. What changes is when In the episodes the Terror Beyond parts one and two Solomon goes on a quest to get back his soul and on the way teams up with an unlikely band of Superman, Hawkgirl, Wonder Woman and Dr. Fate. On their journey Solomon make friends with Hawkgirl and later dies in her arms. Solomons dieing concern is weather he got his soul back. Hawkgirl doesn't believe in souls the way Solomon does but she lies to him so he can die happy. He is later reanimated by some fan-boys in the episode Wake the dead. When he is reanimated he becomes more evil and stronger than before. No one in the Justice League can stop him except Hawkgirl. The reason for this is because of the magical properties of her mace. In the end it is up to her to put him down and in another sad scene she takes him out. The scene is very reminiscent of the book of Mice and men when George kills Lenny. The death however is not shown on camera. Hawkgirl just says she took care of it and he is never seen again.

Powers:

 Grundy has superhuman strength and also endurance. His strength has varied through the years and at one point not even the one and only Superman could restrain him. He is virtually indestructible thanks to the elemental energy that imbues his form with pseudo-life. He is nearly invulnerable to physical and energy attacks and he is not affected by fire or cold. He does not need to eat, sleep or breathe.You could say he is unstoppable.

 Grundy is immortal due to the fact that when he is destroyed/killed he always no matter what comes back to life sooner or later.