Tammy grew up in the city of St. Catharines, Ontario. She was known for her athletic abilities and she avidly participated in a variety of sports, including track and field, cross country running, and soccer, with soccer being her favourite. She was a Grade 10 student at Sir Winston Churchill Secondary School in St. Catharines at the time of her death.
On 24 December 1990, less than two weeks before Tammy's 16th birthday, Karla Homolka and her fiancé, Paul Bernardo, plied Tammy with alcoholic drinks laced with Halcion, a sedative. When she fell unconscious, the two raped her. Tammy Homolka became sick while sedated and died. After failed attempts to revive her, Karla covered up evidence of the assault and called an ambulance. Her official cause of death was listed as choking to death on her own vomit, and was believed at the time to have been an accident.
After Paul Bernardo and Karla Homolka were arrested for the murders of Leslie Mahaffy and Kristen French, Tammy Homolka's body was exhumed and Bernardo was subsequently tried and convicted for her murder.
In September 1990, as their relationship progressed, Bernardo started to press Homolka to allow him to have sex with Tammy. She eventually conceded and obtained a bottle of Halothane (a liquid general anesthetic) from the veterinary clinic where she worked, that would later be used to sedate Tammy during the rape. On December 23, 1990, Tammy was drugged with a combination of Halcion and alcohol, passing out in the family room in the basement of the Homolka house. Homolka's parents and sister Lori were upstairs asleep at the time.
They filmed the entire rape, with Homolka holding the camera, using the Halothane to keep Tammy unconscious.
In 2001, the magazine Elm Street published an article in which it implied that forensic evidence proved that Tammy's death was not an accident and that her sister had deliberately administered an overdose of Halothane. The magazine described Karla as a "malignant narcissist" who was so incensed by her fiance's attraction to her sister that she took steps to remove Tammy from his affections permanently.
When the plea agreement was finalized, the initial 10-year sentence was still intact, with two years added for Tammy's death. She was officially sentenced to 12 years on July 6, 1993.