"This is a reminder to please go get your yearly skin checks," Mellencamp, 40, wrote.
The reality star shared that she initially put off getting her skin checked out of fear but finally went to the doctor after fellow Real Housewives of Beverly Hills star Kyle Richards noticed skin discoloration.
"I had been avoiding mine [because] of my own anxiety for a very long time but when [Kyle Richards] saw my back and it had changed colors again, she immediately took me to the doc," read the post.
The Celebrity Big Brother star chronicled her rough day at the doctor's office on her Instagram Stories as well.
"So Kyle made me go get my moles checked today and well, the day started off with not being able to get the Wordle, now they think I have melanoma. Going to get a biopsy, so check your moles, folks.
She shared photos of her own moles in comparison to cancerous ones, writing, "Praying it's not [cancer]. But get your skin checked once a year, folks. I've been ignoring [because] of my anxiety."
She also stated in her Instagram post that doctors are pretty convinced it is melanoma, the most severe type of skin cancer.
"[The doctor] within minutes said it looks like melanoma and did a big removal," she wrote.
In addition to Richards, the daughter of rocker John Mellencamp tagged husband Edwin Arroyave and friend Jenn Leipart, who have previously urged her to get her skin checked out.
"[They] have been telling me for years to go get checked. I waited [too] long [because] of my own fear. Grateful to them for holding me accountable for going today," she wrote.
The reality star appeared to be in good spirits despite such scary news, writing, "I am doing my best to stay present and positive as I head to [film] today. Grateful that I am busy to keep my mind from racing as I await the results," she added.
She also expressed her hope that sharing her experience will help others become more proactive in the health of their own skin.
"I find strength in being honest with you all and hope this helps others. Please let this be a reminder to take action on your own skin," she wrote.
Her comments were flooded with support and well-wishes.
"Surrounding you with love and prayers," read one comment, while others thanked the former RHOBH star for being so open about her journey.
"I am a melanoma survivor! Thanks for bringing awareness to how serious and devastating it can be. Prayers for clean margins!" another commenter wrote.
Want lifestyle and wellness news delivered to your inbox? Sign up here for Yahoo Life’s newsletter