William Baker was born in a rough neighborhood in New York. Abandoned by his father at an early age, William stole and cheated his way through adolescence. After getting expelled from college after a participating in a rigged football game, He was accepted by a gang that ran extortion and protection rackets, adopting 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 found out that his long time girlfriend Marcy Conroy had left him for Vic Rollins, another member of his gang. In a rage he took it upon himself to exact revenge on Rollins and start a lone crime spree. He was captured and placed in a maximum-security prison at Ryker's Island but escaped through an unguarded drainpipe. He was driven to the east coast by the FBI and various law enforcement organizations.
He hid out in a nuclear test site near Savannah, Georgia, and while on a nearby beach an experimental reactor's steam system exploded, bombarding him with a massive dose of radiation and apparently bonded his body with the sand. Now able to transform into a sand like substance he could easily elude authorities, the Sandman embarked on a criminal carrier that lead him into conflict with Spider-Man, the Fantastic Four, Hulk and many other heroes.
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. 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 decided to reform, adopting his true name and using his gift to help people. Though there was a period of suspicion as to whether his intentions were noble, he gained the public's trust and joined the Wild Pack and a reserve member for the Avengers.
After he helped the Avengers defeat the sorceress Morgan LeFay, Baker again became the man he had grown to despise, Flint Marko. He was brainwashed by the Wizard into believing he had faked his well intentioned heroics and joined the Sinister Six.
Recently, he was poisoned by a bite from Venom, and as a result, 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.
