Pamela Isley was born with a skin condition - an aversion to sunlight. Though the doctor had prescribed her a lotion to protect her, little Pamela's abusive father had forbade her from going outside. On one occasion, when her mother allowed her to play outside, he had angrily punched her in the face, giving her a black eye. As if nothing had happened, Pamela's mother stepped outside and began gardening. Seeing her daughter there, she explained that she liked gardening because the flowers listen to her, and sometimes, if she stays very quiet, they speak back to her. When Pamela had asked about the black eye, her mother had assured her it was nothing to worry about. The abuse continued. Every time he hit her, Pamela's father bought his wife's forgiveness with flowers, and her garden would grow. That was how Pamela learned that flowers could manipulate people. However, she also learned of human cruelty when her father murdered her mother, burying her under her own garden before being arrested.
In college, Pamela created designer drugs, doling pheromone pills out in order to study their effects. The University learned of her activities, and she was expelled and put in jail pending a criminal indictment. However, she had already used her pills to get the Dean under her thumb. Charges were dropped, and she graduated Summa Cum Laude. After graduating, Pamela visited her father in prison for the first time since the murder. He had stubbornly sat in silence, just staring at her and waiting for her to make the first move. After a time, she decided to do what made the most sense. She leapt up out of her seat and kissed him. By the next morning, he was dead, and the toxin used to kill him was untraceable.
The Dean secured Pamela an internship at Wayne Enterprises straight out of College at their R&D site just after the merger with Kane Chemical. She helped to develop pharmaceuticals and cosmetic applications, but she had greater aspirations.
The Zero Year
Pamela became a research assistant at Wayne Enterprises, helping a man named Dr. Paji create a project that would allow plants to grow at an incredibly advanced rate. During the event that would be dubbed "Zero Year" by Bruce Wayne, a terrorist known as the Riddler partnered with a scientist named Dr. Karl Helfern to steal Pamela's experiment. The Riddler turned off the power in Gotham City, turning the city into a savage cesspit. The villain then spread Pamela's project throughout the city, turning the once proud and advanced city of Gotham into a literal urban jungle because of the incredible plant growth. However, both the Riddler and Hellfern were defeated by the new hero known as the Batman, and Gotham City began a rebuilding process, getting rid of all the plants that Pamela's stolen research had caused.
Becoming Poison Ivy
With the city back to normal, Bruce Wayne came to look in on Pamela's division when she took the opportunity to present him with a business project that would cut his advertising division by 100%. In fact, she had used pheromones to get the meeting with him, and this was her proof that Wayne Enterprises could use pheromones to target clients, and modify their behaviour to the company's benefit. Wayne refused her pitch on the grounds that it amounted to brainwashing, and removed free choice from the equation. Ethically opposed to her idea, Bruce Wayne fired her on the spot, warning that his company owned any research she had done, so she would not be able to pitch her idea elsewhere.
Angrily, she had rushed past security to steal what she could of her work back, specifically a plant serum that she had made from all of her knowledge in botanical research, which she alone knew how to make. Unfortunately, in the process, she was doused with her own chemicals. The chemicals changed her, making her immune to poisons and viruses, giving her command over any plant, and connecting her inexorably to The Green as well as giving her immunity to poisons and viruses and innate control of pheromones. She became something of an eco-terrorist, using her abilities to strike back at those who struck out at the Green.
Birds of Prey
Ivy still lived in Gotham City, but had been asked by Black Canary to join the Birds of Prey, despite her tendency toward criminal activity. Ivy accepted, but her new teammates Starling and Katana weren't very happy about it. They grew to trust her, begrudgingly, while investigating a mind-controlling criminal called Choke, who was using unsuspecting people as sleeper agents, and implanting them with bombs triggered by mnemonic queues.
After a battle with one of the Court of Owls' Talons, Ivy sacrificed her health in order to trap him within the refrigerated car of a train, knowing that only severe cold could stop him. She had made Black Canary promise to take her to the Amazon Basin, should she ever become injured, and following the Talon's capture, the Birds of Prey made good on that promise, but were attacked by plant-monsters when they arrived. As it turned out, Ivy had betrayed them, having secretly poisoned them, and intended to coerce them into helping her in her eco-terrorist crusades against polluting corporations. When it became clear that Batman could cure their poisoning, Katana near-fatally injured Ivy, resulting in her being dropped from the team.
A free-agent in Gotham, Ivy broke Clayface out of Arkham Asylum, manipulating him into becoming her husband - and therefore her bodyguard. Batman intervened for her sake, mainly because the locations she attacked were the Penguin's properties, and he had put a bounty on her head. Unfortunately, Ivy was captured by Penguin's men, and buried alive. However, she survived long enough for Penguin's over-ambitious second-in-command Ogilvy to rescue her as part of his plan to take over Penguin's business, since his boss had gone missing thanks to the return of the Joker. She repaid her debt to him by supplying him with a means to chemically enhance himself enough to become a match for the Batman, which ended up giving his skin a consistency akin to that of bark.
When the Crime Syndicate took over the world and seemingly killed the Justice League, the villains of Gotham took rule over it. Poison Ivy, among them, took control of a section of Gotham for herself, primarily being concerned with her plants. When Bane returned to Gotham in order to claim it for himself, Ivy joined Scarecrow's army of Arkhamites to combat Bane's battalion of prisoners from Blackgate Penitentiary in what would later be named "the Arkham War".
Participating in several battles against the Bane's army, Scarecrow eventually pulled out all the stops in order to win the city by secretly drugging all the Arkhamites, including Isley, with a derivative of Bane's precious Venom, but, despite their temporary super strength, the Arkhamites were still defeated by Bane, who declared himself the victor of the Arkham War.
After the fall of the Crime Syndicate and return of the Justice League, Poison Ivy began operating with the Calabrese crime family, being led by her friend Selina Kyle.