In a statement on his Twitter account on June 23, the 49-year-old said he has been receiving chemotherapy treatment for the past three months.
"I have cancer," Mark wrote. "It sucks and I'm scared, and at the same time I'm blessed with incredible doctors and family and friends to get me through this."
"I still have months of treatment ahead of me but I'm trying to remain hopeful and positive," he continued. "Can't wait to be cancer free and see you all at a concert in the near future."
And during a recent Twitch livestream, Mark answered some questions from fans about his experience with chemotherapy.
"I feel much better than yesterday," Mark said, according to Absolute Radio.
"Yesterday was hellish for me and I woke up today feeling better," he went on. "I went for a walk, and I had a decent breakfast, and I haven't felt like I was going to throw up today, so we'll take it as a win."