Mark Jensen was born on October 26, 1959. He met Julie Griffin in 1981 while both were working and attending college in Oshkosh, Wisconsin. They married and eventually settled in Pleasant Prairie, Wisconsin, raising two sons, David and Douglas. At the time of Julie's death, she was 40 years old and working part-time for the Port Authority in Chicago. Mark worked as a branch manager for a financial firm based in St. Louis. On the surface, they seemed like an ordinary suburban family. However, cracks began to show in their marriage—Julie had previously had a brief affair, and Mark had begun a new relationship with a co-worker named Kelly LaBonte, whom he later married. Julie expressed growing fear for her safety, documenting her suspicions in a letter given to a neighbor in case something happened to her. She made it clear that she would never take her own life and urged police to investigate her husband if anything happened.

On December 3, 1998, Julie Jensen was murdered in her Pleasant Prairie home. According to prosecutors, Mark Jensen poisoned her with ethylene glycol, the main ingredient in antifreeze, and then suffocated her while she lay incapacitated in bed. Mark had allegedly researched poisoning methods online and discussed it with coworkers and a jailhouse informant. Julie’s suspicions were confirmed posthumously when a letter she wrote—predicting her death and naming Mark as a suspect—was submitted as evidence.

The investigation gained national attention, especially due to the legal controversy surrounding the letter. In 2008, after a lengthy trial, Mark was convicted of first-degree intentional homicide and sentenced to life in prison without parole. However, in 2013, a federal court ruled that admitting the letter into evidence violated Mark's Sixth Amendment right to confront his accuser. His conviction was overturned, and after years of appeals and legal maneuvering, a second trial began in January 2023. The letter was excluded, but new forensic and testimonial evidence convinced another jury to find him guilty again on February 1, 2023. He was sentenced a second time on April 14, 2023, once again to life in prison without the possibility of parole.