In the process, he also shared that he's cancer-free.
"Half of my life I was focused on the aesthetics of my body and about half of my career I spent shirtless... now I proudly bear the scars that made me cancer free. #scarstoyourbeautiful #cancerfree #grateful," he wrote.
In the candid image, the TV host and actor lifts his shirt to show what doctors had to do to remove the tumor.
This amazing news comes a week after his surgery.
Last Friday, he posted a photo of himself from the hospital, "Feeling loved and supported by my family and friends, including each and every one of you. I’ve been very overwhelmed and so grateful for all of the supportive comments and prayers. The surgery went very well. The tumor is gone and I even got to keep 80% of my kidney We are all optimistic."
In an interview with "GMA" last week, Mathison said that after years of pain and knowing something was wrong, he made the doctors prescribe him an MRI. That imaging found the mass.
After he found out about the tumor that had been growing a minimum of 10 years, he called his wife first.
"First thing out of her mouth, which is amazing to me is, 'We got this, we're going to beat it,'" he said.