Kelley Mack, who was best known for her role as Addy on the hit TV series “The Walking Dead,” died in Cincinnati last weekend, her family announced on Tuesday. She was 33.

The cause was an aggressive tumor that attacked her central nervous system, her family said in a statement.

In January, Ms. Mack announced that she had been diagnosed with an astrocytoma, a kind of cancerous tumor that grows in the central nervous system, beginning in the brain or the spinal cord. It is also known as diffuse midline glioma.

An astrocytoma is formed by the abnormal growth of astrocytes, star-shaped cells in the nervous system that help the nerves function.

Scientists are not certain of the cause of abnormal astrocyte growth, according to the National Cancer Institute.

While diffuse midline gliomas are mostly found in children, they can occur in adults as well, according to the Brain Tumor Center at the University of California, San Francisco.

Ms. Mack wrote in an Instagram post in January that she had started experiencing persistent lower back pain, which led her to believe she had a slipped disk.

“Then, the shooting pains in my legs and back began, which resulted in me having to sleep in a recliner for a month because laying down was too painful,” she wrote.

Ms. Mack had an emergency M.R.I. scan, and doctors found “an abnormal mass” in her spinal cord, she wrote. After a biopsy surgery on her spinal cord, she said, she lost most of the use of her legs and had to use a wheelchair and a walker.

“It has been a very emotional and challenging time, truly testing my mental fortitude, faith, and physical strength,” Ms. Mack wrote.

After the surgery, Ms. Mack underwent radiation treatment in Cincinnati.

In March, Ms. Mack posted a video of herself walking up a set of stairs. “It’s a big deal for me,” she wrote. “Life has also had its ups and downs lately.” In recent months, she also posted updates about “Universal,” an independent film in which she had starred.

Ms. Mack was born Kelley Lynne Klebenow on July 10, 1992, in Cincinnati. She graduated from the Dodge College of Film and Media Arts at Chapman University in Orange, Calif., in 2014.

She is survived by her parents, Kristen and Lindsay Klebenow, and her siblings Kathryn and Parker.

She earned 35 acting credits, including Addy in the ninth season of the AMC series “The Walking Dead.” She also appeared on the NBC drama “Chicago Med” and the Fox show “9-1-1,” among others.

“Kelley’s love for storytelling blossomed at a young age, when she received a mini video camera as a birthday gift,” according to her family’s statement. “This sparked a lifelong passion for the arts,” leading Ms. Mack to start acting in commercials when she was a child.