Mitchell was diagnosed with ALS, a neurodegenerative disease, more than five years ago, according to a statement on his Instagram page. According to the statement, he faced "terrible challenges" during those years.
"And in true Ken fashion, he managed to rise above each one with grace and commitment to living a full and joyous life in each moment," the statement said. "He lived by the principles that each day is a gift and that we never walk alone."
ALS, also known as Lou Gehrig's disease, affects the nerve cells that make muscles work in both the upper and lower parts of the body, according to the Centers for Disease Control and Prevention. The disease makes the nerve cells stop working and die.
Mitchell first publicly spoke about his diagnoses in a 2020 interview with PEOPLE: "The moment that they told us it was ALS, it was like I was in my own movie. That's what it felt like, like I was watching that scene where someone is being told that they have a terminal illness. It was just a complete disbelief, a shock."
Mitchell appeared in several roles after his diagnosis. He played the Klingons Kol, Kol-Sha, Tenavik and Aurellio in Star Trek: Discovery. He also voiced several characters in an episode of Star Trek: Lower Decks.
Mitchell was also known for his work in the series Jericho and The Astronaut Wives Club and his work in the films Miracle and Captain Marvel. He played his last (series) role in 2022 as a guest actor for three episodes in The Old Man.
In the Instagram post announcing his death, Mitchell was remembered as a daydreamer, naturalist, cat lover and "more than anything, a proud father."
Mitchell is survived by his parents, his wife and their two children. According to StarTrek.com, Mitchell asked that donations be directed toward ALS research or toward his children.