Born into a long line of Irish New York police officers, Patrick Mulligan was living his family's dream. Recently married to a loving woman named Gina, the couple had a baby on the way, and his NYPD career was going perfectly.

However, Patrick didn't expect a chance encounter with the psychotic Carnage, who was enraged at his own impending asexual pregnancy. Carnage implanted Patrick with his "other," and left the new Symbiote to incubate within him. At the same time, Venom argued with Carnage over the Symbiote offspring. Carnage insisted on killing it, but Venom was concerned that as the 1,000th Symbiote of their line, "Toxin" could potentially become psychotic as a result of genetic breakdown. Carnage tracked his offspring to Patrick's apartment, and threw Gina down a stairwell. Spider-Man was able to save Gina, while Black Cat managed to distract Carnage.

Patrick continued his life, haunted by his encounter with Carnage. He soon discovered that he was much faster and stronger than before, and was easily able to subdue Black Cat before she could escape his questioning. However, Carnage interrupted the conversation, attacking Patrick and the Cat. Patrick became Toxin, saving Cat. Though unnerved by his transformation, Patrick decided that his new powers could be used for good. Teaming up with Spider-Man to stop an armed robbery, Patrick found himself filled with murderous urges. He struggled to keep himself together, telling Spider-Man that he was only "joking" about eating the robbers' brains.

Mulligan later used his powers to save Spider-Man and Black Cat from Venom and Carnage, beating them both single-handedly. He realized that he had much in common with Spider-Man and made the web-slinger his role model. Realizing his presence was putting Gina and newborn Edward in serious danger, Pat left them behind, offering Gina little explanation. Patrick now walked the streets of New York alone, as Toxin, hoping to learn as much as he could from Spider-Man.

A short time after fighting Venom and Carnage, Toxin began helping to find the escaped criminals from the Raft. The first criminal he hunted down was King Cobra. Patrick visited Mr. Hyde at Ryker's Island, hoping to obtain information on King Cobra. Hyde gave no help, but did give a piece of King Cobra's skin he had previously shed. Using the piece of skin, Toxin located Cobra, and after a small scuffle, defeated him. After the battle, Spider-Man came to Pat enlisting his aid to bring in the criminal RazorFist, giving him a bloody bandage to aid in Toxin's hunt.

At this point in his life it was becoming continually more difficult for Mulligan to control the symbiote. Toxin liked the violence, and it was getting increasingly harder for Patrick to refrain from killing. Eventually, Toxin was able to locate Razor Fist, but instead of changing into Toxin, he changed into a small, middle-aged man, Toxin called Larry. A twisted joke Toxin decided to play on Pat for never letting him have any "fun". After Patrick pleaded for the symbiote's help, he eventually emerged. After a short fight, RazorFist escaped. Toxin then defeated and captured the criminal known as The Answer. Patrick could think of only one to solution the childlike monster that brought out his worst: suicide.

Mulligan attempted to jump in front of a subway train, but was saved by the symbiote (who viewed this as "a cry for help"). After the attempted suicide, Toxin brought down the Wrecker and Piledriver, two members of the criminal group known as the Wrecking Crew. Patrick and Toxin made an agreement to allow Pat two hours a day to become Larry and see his family, and Toxin would in turn get two hours late at night for "playtime". Conditions of the agreement being no acts of arson, grand theft, or homicide. RazorFist killed Mulligan's father in an attempt to gain information about Patrick. Toxin found and beat RazorFist, taking his razor fists, and bringing him to justice.