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Julie Jensen, a 40-year-old mother of two, was found dead in her Pleasant Prairie, Wisconsin, home on December 3, 1998. Her husband, Mark Jensen, poisoned her with antifreeze, drugged her with sleeping pills, and suffocated her. He was later convicted of first-degree intentional homicide.
The case is featured in Dateline NBC Season 32, Episode 4, titled Secrets in Pleasant Prairie, which originally aired on October 12, 2023. Dateline will re-air Secrets in Pleasant Prairie on April 19, 2025, at 9 pm ET/PT on NBC.
It was also featured in 20/20 season 46, episode 29, titled The Kill List, which premiered on February 10, 2023, and had an encore on December 27, 2024. The episode is available for streaming on Hulu.
Julie Jensen’s murder involved a chilling plot driven by Mark’s affair and resentment over her past infidelity. A letter she wrote, suspecting her husband, became pivotal, leading to two trials and convictions over 25 years.
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5 Details Related to the Murder of Julie Jensen
1) Julie wrote a letter suspecting Mark would kill her
According to NBC News, on November 21, 1998, Julie Jensen wrote a letter stating:
“If anything happens to me, he would be my first suspect.”
She gave it to neighbors Ted and Margaret Wojt with instructions to hand it to the police if she died. The letter detailed her fear of Mark, referencing his unforgiveness for her affair seven years earlier and his suspicious behavior, such as urging her to drink more.
She also included a photo of Mark’s shopping list, which featured syringes and chemicals. The letter, discovered a day after her death on December 3, 1998, became key evidence. Despite legal battles over its admissibility, it significantly shaped the case, although it was excluded in the 2023 retrial. Julie’s words ultimately guided investigators to Mark.
2) Mark poisoned Julie Jensen with antifreeze over days
According to NBC News, Mark Jensen began poisoning Julie Jensen with ethylene glycol, the main ingredient in antifreeze, in late 1998. Toxicological tests after her death on December 3, 1998, revealed antifreeze in her blood, stomach, and urine.
Prosecutors stated Mark mixed it into her food or drink, leading to a slow, painful decline. Julie had complained of breathing issues and weakness for several days.
He also administered sleeping pills to further weaken her. On the day of her death, Mark suffocated her with a pillow while she was barely conscious. Internet searches on their home computer from October 1998, including ethylene glycol poisoning and toxicology, linked Mark to the crime.
This methodical approach showed premeditation.
3) Mark’s affair with a co-worker was a motive
According to WISN reports on January 17, 2023, Mark Jensen had an affair with Kelly LaBonte, a married co-worker, prior to Julie’s death in 1998. Emails recovered from his devices revealed expressions of affection and plans to start a new life with LaBonte, whom he later married.
Prosecutors argued that the affair was the motivating factor for Mark to kill Julie Jensen instead of divorcing her.
Julie’s letter mentioned their strained marriage, citing her own brief affair years earlier, which Mark never forgave. During the trial, prosecutors highlighted Mark’s resentment and desire to be with LaBonte as driving forces. LaBonte assumed custody of Mark and Julie’s sons, David and Douglas, after Mark’s imprisonment.
4) A jailhouse informant revealed Mark’s confession
According to Law & Crime reports, dated February 1, 2023, in 2007, while in Kenosha County Jail, Mark Jensen allegedly confessed to inmate Aaron Dillard about Julie Jensen’s murder. Dillard testified that Mark admitted poisoning Julie Jensen for three days, and when she seemed to recover, he rolled her onto her side and suffocated her with a pillow.
Dillard’s testimony included specific details about the method of suffocation, which prosecutors argued only the perpetrator would know.
This confession bolstered the case during the 2008 trial and was considered during the 2023 retrial. Despite defense claims that Dillard sought a lighter sentence, his testimony aligned with forensic evidence and Julie’s letter.
The informant’s account helped secure Mark’s conviction, showing his willingness to share his crime.
5) Two trials convicted Mark after legal battles
According to NBC News, Mark Jensen was first convicted on February 22, 2008, after a six-week trial in Walworth County. He was sentenced to life without parole on February 27, 2008. The conviction relied heavily on Julie’s letter, but in 2013, a federal judge overturned it, citing a violation of Mark’s right to confront his accuser.
The Wisconsin Supreme Court ordered a retrial in 2021, barring the letter and voicemails.
The second trial began January 9, 2023, and on February 1, 2023, a jury convicted Mark again after one day of deliberation. On April 14, 2023, Judge Anthony Milisauskas sentenced him to life without parole. Mark, now 63, is at Oshkosh Correctional Institute.
Stay tuned for more news and updates, and watch Secrets in Pleasant Prairie as it re-airs on Dateline on April 19, 2025.
