The entertainment world mourns the loss of Kelley Mack, a talented actress known for her work on “The Walking Dead,” “Chicago Med,” and “9-1-1,” who passed away on August 2, 2025, at just 33 years old. Mack died peacefully in her hometown of Cincinnati, surrounded by her loving mother Kristen and aunt Karen, after a brave battle against diffuse midline glioma, an extremely rare and aggressive form of brain cancer. Her story highlights both a remarkable life cut short and the devastating impact of rare cancers that increasingly affect young adults.
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A Life Dedicated to Storytelling
Born Kelley Lynne Klebenow in Cincinnati, Ohio, Mack held 35 actress and five producer credits during her career. She graduated from Chapman University in 2014 with a bachelor’s degree in cinematography and had been living and working in Los Angeles for the past decade. Her passion for storytelling began early when she received a mini video camera as a birthday gift, sparking a love for filmmaking that would define her career.
Mack was best known for playing Addy in season 9 of “The Walking Dead,” and Penelope Jacobs in season 8 of “Chicago Med” and on FOX’s “9-1-1”. She also worked extensively in commercial and voice-over work, appearing in advertisements for major brands including Dr. Pepper, Ross stores, Dairy Queen, and Chick-fil-A.
The Devastating Diagnosis
In January 2025, Mack publicly shared her health struggle, revealing that after moving in with her boyfriend in September 2024, she began experiencing persistent lower back pain, initially thinking she had slipped a disc. The symptoms escalated to include “neuropathic itching,” “shooting pains in my legs and back,” forcing her to sleep in a recliner because lying down was too painful.
Emergency MRIs on Thanksgiving Eve 2024 revealed an abnormal mass in her spinal cord, leading to her diagnosis with diffuse midline glioma, an extremely rare type of astrocytoma cancer. “Due to the biopsy surgery on my spinal cord, I have lost the use of my right leg and most of my left leg, so I now get around with a walker and a wheelchair,” she shared in her January Instagram post.
Understanding Diffuse Midline Glioma
Diffuse midline gliomas are primary central nervous system (CNS) tumors that begin in the brain or spinal cord. They are a rare subtype of glial tumors, with all cases classified as grade 4 (grade IV), meaning they are malignant and fast-growing.
While DMGs primarily affect children with a median age of diagnosis around 6-7 years old, they can occur in adults. The estimated survival post-diagnosis remains only 9–15 months. However, the relative five-year survival rate for diffuse midline glioma is 42.2 percent, though many factors can affect prognosis including tumor grade, the person’s age and health when diagnosed, and response to treatment.
An estimated 3,940 people are living with this tumor in the United States. Treatment typically involves radiation therapy as the most effective and common initial treatment, typically done over a 6-week period, sometimes combined with various forms of chemotherapy.
Cancer in Young Adults: A Growing Concern
Kelley Mack’s story tragically illustrates a broader trend of cancer affecting younger adults. About 80,000 young adults aged 20 to 39 are diagnosed with cancer each year in the United States, representing about 4% of all cancers. Cancer is the 5th leading cause of death in this age group.
Common Cancers Affecting People in Their 30s and 40s
Research shows that certain cancer types are more prevalent in younger adults:
In 30 to 39 year olds: breast cancer, thyroid cancer, melanoma, and colon and rectum cancer are the most common cancer types. Even within this age group, some cancers become more or less common as people age. For example, lymphomas are more common before age 25, whereas breast, cervical, and colorectal cancers become more common after age 25.
Cancer Types by Age Group (30-40 Years)
| Cancer Type | Key Statistics | Symptoms |
|---|---|---|
| Breast Cancer | Usually found in women over 40, rare before 30, but 9% of cases occur in women younger than 45 | New lump in breast, changes in breast size or shape |
| Thyroid Cancer | More common in younger individuals than most other cancers, far more common in women than men | Lump in front of neck, voice changes |
| Melanoma | One of the most common cancers in people younger than 30, especially women | New spot on skin, changes in existing moles |
| Colorectal Cancer | Incidence has doubled among people younger than 50 since the 1990s | Changes in bowel habits, blood in stool, abdominal discomfort |
| Lymphoma | More common before age 25 in young adults | Swollen lymph nodes, unexplained weight loss, fatigue |
Risk Factors and Prevention
Experts point to several potential factors contributing to rising cancer rates in young adults, including obesity, highly processed foods, sedentary lifestyles, and environmental exposures. Reproductive factors also play a role, as younger women are experiencing menstruation earlier and having children later, exposing them to more years of unopposed reproductive hormones.
Survival Rates by Cancer Type (Ages 20-39)
| Cancer Type | 5-Year Survival Rate | Treatment Response |
|---|---|---|
| Thyroid Cancer | >95% | Excellent prognosis, especially in young adults |
| Testicular Cancer | >95% | Highly treatable with positive outcomes |
| Breast Cancer | 85-90% (varies by subtype) | Triple-negative and HER2-positive subtypes more common in young women, with worse prognosis |
| Melanoma | 92% (early stage) | Often curable if found and treated early |
| Diffuse Midline Glioma | 42.2% | Estimated survival post-diagnosis: 9–15 months |
Legacy and Impact
“Kelley was known for her warmth, creativity and unwavering support for those around her. Her infectious enthusiasm and dedication to her work inspired countless others in the industry and beyond”, according to her family’s statement. Her colleagues from “The Walking Dead” remembered her as “a bright light on every level” and “a true talent and lovely soul”.
Mack and her mother had collaborated on screenwriting projects, including “On The Black,” a 1950s era project based on her maternal grandparents’ story. She had been promoting her upcoming 2025 film “Universal” on social media in the months before her passing.
The Importance of Early Detection
Cancers in young adults are often found later than they are in other age groups, partly because young adults might not go to the doctor unless they feel they really need to, and cancer is not usually high on the list of probable causes for symptoms in this age group.
Warning signs that should prompt medical attention include unusual lumps or swelling (especially in the neck, breast, belly, or testicle), persistent fatigue, unexplained weight loss, changes in bowel or bladder habits, persistent cough or hoarseness, unusual bleeding or discharge, and changes in moles or skin.
Looking Forward
Kelley Mack’s courageous battle with diffuse midline glioma brings much-needed awareness to this rare cancer. Recent research has shown promise with a compound called ONC201, which nearly doubled survival for patients with diffuse midline glioma compared to previous treatments, offering hope for future patients facing this devastating diagnosis.
A celebration of Kelley’s life was held in Ohio on August 16, 2025. She is survived by her parents, Kristen and Lindsay Klebenow; her sister Kathryn and brother Parker (also an actor); her grandparents; and her boyfriend Logan Lanier.
As her sister Kathryn beautifully stated, “She would want you all to know how much she loves you. And as her sister, I want you all to know how brave that tough SOB was“. Kelley Mack’s story serves as both a celebration of a life well-lived and a reminder of the urgent need for continued research into rare cancers affecting young adults.
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FAQs
What is diffuse midline glioma and how rare is it?
Diffuse midline gliomas are primary central nervous system tumors that begin in the brain or spinal cord. They are a rare subtype of glial tumors, with approximately 3,940 people living with this tumor in the United States. All cases are classified as grade 4, meaning they are malignant and fast-growing.
What are the survival rates for diffuse midline glioma?
The relative five-year survival rate for diffuse midline glioma is 42.2 percent, though the estimated survival post-diagnosis typically remains only 9–15 months. Survival depends on many factors including tumor grade, patient age, and response to treatment.
What are the most common cancers in people aged 30-39?
In people aged 30 to 39, the most common cancer types are breast cancer, thyroid cancer, melanoma, and colon and rectum cancer. About 80,000 young adults aged 20 to 39 are diagnosed with cancer each year in the United States.
Why are cancer rates increasing in young adults?
Experts point to several potential factors including obesity, highly processed foods, sedentary lifestyles, and environmental exposures. For breast cancer specifically, factors include experiencing menstruation earlier and having children later, exposing women to more years of reproductive hormones.
What symptoms should young adults watch for that might indicate cancer?
Warning signs include unusual lumps or swelling (especially in neck, breast, belly, or testicle), persistent fatigue, unexplained weight loss, changes in bowel or bladder habits, persistent cough or hoarseness, unusual bleeding or discharge, and changes in moles or skin. Any persistent symptoms should be evaluated by a healthcare provider.
