The 28-year-old made the announcement in a story he wrote in The Players' Tribune.
He wrote that he had a port put into his chest on April 6 and began chemotherapy on April 13 to treat stage 3 colon cancer. He is scheduled to receive the treatment every two weeks for six months.
Getting "chemo in the age of COVID-19 is crazy," he wrote, describing the added challenges of getting cancer treatment during the coronavirus pandemic.
"I'm getting chemo at a hospital in Baltimore, and I have to drive up there alone," he wrote. "Nobody is allowed to come in with me, and that's completely fine by me. I don't want anybody else being put at risk -- people that are close to me and that I love, and other people in the hospital. You just never know. COVID-19 has spread so quickly. I'm definitely trying to follow all the protocols, not only because it's the right thing to do, but also because I don't want to expose myself to anything, especially before going into chemotherapy."
He added: "If baseball returns in 2020, it will probably be without me."
Mancini said that after his team physical at spring training, doctors informed him his blood tests showed he had low iron levels. He had an endoscopy and colonoscopy on March 6 and had surgery on March 12 to remove a malignant tumor from his colon.
"I know that this is a terrible time for everybody," Mancini said. "So many people have lost jobs, so many people have lost loved ones. After my chemo is done, and when I'm totally cancer-free, I've got a few different ideas of what I can do. I'm lucky enough to have a platform that I feel allows me to make a difference for some people -- even if it's just spreading awareness about the importance of getting a physical every year."
He was one of the few bright spots for the Orioles last season, hitting .291 with 35 home runs and 97 RBIs.
"Really, the support I've gotten from everyone has just been unbelievable," Mancini wrote. "It's given me an appreciation for a lot of things that I've always had, but that were getting overlooked as I went about my day-to-day life. Going through something like this had really made me understand all my blessings."