William Baker was born in a rough neighborhood in New York. Abandoned by his father when he was three-years old, William stole and cheated his way through adolescence. Expelled after accepting a bribe to throw a football game, he was accepted by a gang that ran extortion and protection rackets and adopted the alias Flint Marko.
He soon became very successful in the New York criminal underworld, but was arrested, convicted, and forced to spend many years in solitary confinement. Upon his released he discovered that his long time girlfriend Marcy Conroy had left him for Vic Rollins, another member of his gang. In a rage he got revenge on Rollins and started a lone citywide crime spree. Marko was captured and placed in a maximum-security wing at Ryker's Island prison but escaped through an unguarded drainage tunnel. With the FBI and police on his tail, Marko traveled south and sought refuge in a place where he was unlikely to be found: an atomic testing site near Savannah, Georgia. As he rested on a nearby beach, the experimental reactor's steam system exploded, bombarding him with a massive dose of radiation and apparently bonding his body with the sand. Awakening after a brief period of unconsciousness, Marko discovered his newly acquired ability to transform into a sand-like substance he could manipulate and reshape at will. Now easily able to elude authorities, the Sandman was born.
The Sandman embarked on a criminal career that led him into conflict with Spider-Man, the Fantastic Four, Hulk, and many other heroes. After failing to defeat Spider-Man as a member of the Sinister Six under Doctor Octopus, he joined forces with the Wizard who provided him with a belt that enabled him to produce a variety of dangerous and deadly effects through the injection of various chemicals. The Sandman and the Wizard then teamed with the Trapster to become the core members of the original Frightful Four. The team encountered many defeats at the hands of the Fantastic Four and they soon broke up. Marko later broke out of prison with the Wizard's help, and broke into the Baxter Building, briefly teaming up with Blastaar when he first escaped from the Negative Zone.
Marko's criminal career was struck a major blow when he joined forces with Hydro-Man and in a freak mishap combined to form a grotesque, mud-like monster called the "Mud-Thing". The creature was subdued by the authorities and transported to a laboratory in Manhattan were scientists sought to analyze the composition of the unusual mud, but determined with the instruments at their disposal that it was inanimate. They decided to seal the substance in a container and dispose of it. As a delayed effect of the analytic procedures, the mud spontaneously separated into the two criminals, who burst free of their container.
The ordeal so traumatized Marko that he was stricken with a troubled conscience. Striking up a surprising friendship with the Thing of the Fantastic Four, the Sandman soon talked of abandoning crime. When he encountered Spider-Man fighting the Enforcers, the Sandman made good on his intentions and rescued the wall-crawler. Still a wanted man, the Sandman adopted the alias Sylvester Mann. He rented a room in Coney Island from a suburban family and decided to reform earning money as a contract worker for Silver Sable as a member of her Wild Pack. He joined the Outlaws for their brief time in existence, aiding Spider-Man. Though there was a period of suspicion as to whether his intentions were noble, he gained the public's trust and became a reserve member for the Avengers along with Spider-Man. Granted a pardon for his crimes, he returned to his birth name of William Baker. After he helped the Avengers defeat the sorceress Morgan Le Fay and stop the theft of military equipment, a minor misunderstanding with Captain America caused by his undisciplined temperament lead to his resignation from the reserve Avengers. However, the Sandman was determined to remain reformed. When Doctor Octopus blackmailed him into rejoining the Sinister Six, the Sandman turned against his former ally as soon as he had the chance.
Feeling the loss of his partner-in-crime, the Wizard kidnapped Baker and forcibly subjected him to a machine that amplified the underlying dark side of its subject's personality. The violent Flint Marko regained consciousness, and believed he had faked his well-intentioned heroics. He was not to be the Wizard's lackey.
The now-villainous Sandman re-formed the Sinister Six to seek revenge on Doctor Octopus, but Venom, turned on his teammates and took a large bite from Sandman. Weakened and poisoned by Venom, Baker began to lose bits of himself until he eventually dissipated. Delirious from his condition, he sought out Spider-Man to exact his final vengeance. In his weakened state he was unable to defeat the wall-crawler. With his last bit of strength, he made peace with Spider-Man and asked that he say goodbye to his mother for him. The despondent Sandman then lost cohesion and was washed away into a New York City storm drain.
The Sandman's dispersed body was eventually washed up onto a beach, where his consciousness remained in a coma-like state. Eventually, a television rock concert awoke Sandman, who began to attack the concert-goers. Eventually, the Sandman gathered enough strength to re-form his body, but his mind was still heavily damaged.
The Sandman began to split into different versions of himself - his good side, his evil side, his inner child, and his feminine side - each taking on a different aspect of his emotions. With the aid of Spider-Man, the Sandman was able to reabsorb his duplicates and returned to his single, villainous form. However, Sandman once again partnered with Spider-Man. With his father Floyd Baker facing imminent execution for the murder of a homeless vagrant (described as bearing the likeness of Spider-Man's long-dead "Uncle Ben") the Sandman worked to rescue or exonerate his father. Spider-Man and the Sandman discovered the true murderer was the monstrous Chameleon of the future year 2211 from Earth-9500. Floyd Baker narrowly missed execution by the electric chair, and was exonerated (though ungrateful of his son's efforts).