Jennifer San Marco was born in Brooklyn, New York, to Frank San Marco and Jeanette San Marco. She attended Edward R. Murrow High School before going to Brooklyn College. Later, she studied natural resources management at Rutgers University, but she did not graduate from there.

In 1989, Jennifer moved to California. After her studies, she was hired as a guard at Chuckawalla Valley State Prison, a medium-security facility in Blythe. However, she quit just two days before her probationary period was over. She did not give a reason for leaving, but people described her as a good worker.

Over the years, Jennifer held several jobs. In the mid-1990s, she worked as a dispatcher for the Santa Barbara Police Department. This position required a background check and psychological evaluation. She left that job after a few months. Next, she worked at a high school serving lunch, but quit in 2000. Eventually, she purchased a condominium and started working as a clerk for the Postal Service.

In 2003, Jennifer had an incident at work where she got pulled out from under a mail-sorting machine. Police had to wheel her away in handcuffs. After this, she took a psychological disability leave. Although she returned briefly, she was removed from her workplace again due to erratic behavior and did not go back.

In February 2001, Jennifer stayed at a mental hospital in Ventura for three days. In 2004, her car broke down in Grants, New Mexico, where she decided to remain. In this small town, she developed a reputation for strange behavior. This included speaking or yelling to herself, ordering food and leaving before eating, and stripping in public. She often prayed in random locations, talked to an imaginary friend, and made alarming comments.

Jennifer began getting tattoos and showed them to a neighbor. She attempted to start a periodical named The Racist Press but was denied a business license. After that, she frequently visited the office, where her behavior caused concern. At times, she made rude comments to others, and there were incidents where she was nearly arrested for her public behavior.

Jennifer sold her condominium and claimed she was visiting a sister on the East Coast. In November 2004, a manager from a mental health clinic reported seeing her talking to herself in a post office parking lot. When questioned, she mentioned that "they pray before they get in," referring to her sister and brother, who she believed were there. A propane gas delivery man described her as polite but noted her tendency to talk to an imaginary friend.

On January 30, 2006, Jennifer San Marco committed a series of murders in California. She started by driving to her former condominium in Santa Barbara, where she killed Beverly Graham, a neighbor with whom she had previously argued. After that, she drove to a mail processing plant in Goleta. To gain access to the facility, San Marco followed another car through the gate. Once inside, she took an employee's identification badge at gunpoint and ordered the worker to leave unharmed.

At around 9:00 p.m., San Marco shot Ze Fairchild in the head in the parking lot. She then turned and shot Maleka Higgins, followed by Nicola Grant. Some employees heard the gunfire and looked out of the windows, at which point San Marco reportedly smiled at them before continuing her attack. Many workers fled to a nearby fire station for safety.

San Marco then entered the building where around eighty people were present. She shot her former supervisor, Charlotte Colton, in the head. Colton survived for two days in the hospital before dying from her injuries. San Marco found Guadalupe Swartz at her old workstation. When Swartz tried to escape, San Marco shot her multiple times. Dexter Shannon, who was wearing headphones at the time, also became a victim at close range.

After the shootings, Jennifer San Marco took her own life by shooting herself in the head. Authorities later linked Graham's murder to the events at the post office. The tragedy resulted in a significant outpouring of grief, with over 1,200 people attending Colton's funeral. The mayor of Goleta condemned the violence and offered condolences to the families of the victims.