Johnson said in an interview with "Good Morning America" co-anchor Michael Strahan that he was diagnosed with the disease last year, at age 39.
"There's no history of ALS in my family," Johnson said in the interview that aired Monday on "GMA." "My doctors believe my case is what's called sporadic ALS, which is actually how the vast majority of ALS cases happen."
He continued, "That's one of the reasons this disease can be so shocking. It can happen to someone who never expected it."
Johnson, who was selected in the first round of the 2008 NFL draft by the Tennessee Titans, said he was in the prime of his life last year, working out daily and spending time with his wife and four children, when he noticed the first symptom of what he later learned was ALS.
"I first noticed weakness in my right hand," he said. "At first, it was little things like my grip didn't feel right and I wasn't as strong as I've always been."
Johnson's wife, Brittany Johnson, said she thought his hand weakness was the result of his years spent on the football field. The running back ran over 11,000 total offensive yards in his NFL career, before retiring in 2017.
"I thought because of football and, you know, his career, that it had to be something with that," she told Strahan. "Maybe ... a pinched nerve or something along those lines, but never ALS."
ALS, short for amyotrophic lateral sclerosis, is a degenerative neurological disorder that eventually leads to paralysis, taking away a person's ability to move, speak, swallow or even breathe, according to the National Institutes of Health.
Researchers have studied possible links between ALS and factors such as injury or physical activity, but no confirmed cause has been found for most cases.
There is currently no known cure for ALS, but some treatments may slow down the progression of the disorder and improve the quality of life for patients with ALS. The NIH states that most people with ALS die from respiratory failure, usually within three to five years of symptoms first appearing, though about one in 10 people with the condition live 10 years or more.
Chris Johnson described receiving the diagnosis of ALS as a "shock," revealing that the doctor who diagnosed him said a medication may extend his life by a few months and that he and his wife should "get our affairs in order."
"Honestly, I don't know if you ever fully process it," he said of the diagnosis. "At first, you're in shock. Then you realize you have two choices. You can give up, or you can fight. I chose to fight."
The former NFL star said his journey with ALS changed after watching an interview last year featuring ABC News' Diane Sawyer and the late actor Eric Dane, who died due to ALS in February, along with Dane's doctor at the time, Dr. Merit Cudkowicz, a neurologist and leading ALS researcher.
"After watching 'Good Morning America' and seeing Dr. Merit with Eric Dane, we reached out to her," Chris Johnson said. "She was willing to think more creatively, offering experimental treatments that might help and advance research."
He continued, "We've been with her ever since, and that changed the course of our fight."
Cudkowicz, professor of neurology at Harvard Medical School and executive director of the Mass General Brigham Neuroscience Institute, told "GMA" she believes Johnson's participation in a clinical trial as part of his care may have helped in his fight against ALS.
"Chris has been on standard of care, which is about three different medications that slow down the illness," she said. "But he was also part of a clinical trial, a therapy that decreases inflammation, and I think that helped him a lot."
Still, in Chris Johnson's case, the disease has progressed so rapidly that he now uses a speech-generating device to speak. Soon after his diagnosis, his voice was recorded, so his speech-generating device delivers a voice that sounds just like his.
"It's continued to progress much faster than I ever imagined. I want people to understand just how quickly ALS can attack your body," he said. "Just over a year ago, I was picking up my 7-year-old daughter so she'd make a wish with her birthday cake. Today, I couldn't do that."
Chris Johnson added that his wife and their children have kept him going through the medical obstacles since his diagnosis.
"She hasn't left my side through any of this," he said of Brittany Johnson. "My kids are also a huge part of why I keep going. Every day I wake up wanting more time with them to make more memories and just be their dad. They give me a reason to keep fighting."
And for Brittany Johnson, though caregiving is "tough," she said she doesn't doubt supporting her husband through it all.
"Our life has shifted so much. It's a heavy workload," she said. "But I have no doubt that this is what, you know, I was called to do."
The Johnsons said they are sharing their story to raise awareness of ALS and the need for ALS research.
An effort to support ALS research in honor of Chris Johnson has been established at the Sean M. Healey & AMG Center for ALS, which Cudkowicz, his doctor, leads.
"I can't even hold a cup if I try, and that's despite being diagnosed relatively early and doing everything we can, including participating in multiple experimental treatments," Chris Johnson said. "That's why early detection, more research, and better treatments are so important. We have to give people a better chance than what's available today."
He said he not only wants to share his story to raise awareness of ALS, but also to let people know that he is still the Chris Johnson they know from his NFL days and beyond.
"I want people to know that I'm still me. ALS has changed what my body can do, but it hasn't changed who I am," he said. "People sometimes look at the physical disability and assume you're not still the same person inside. I still think the same. I still dream. I still love my family. My body just doesn't cooperate."