The father of two from Ballymoney was diagnosed with a rare stomach cancer after nurses discovered his iron levels were low when he went to give blood in November 2012.
Chris, who works as an electrical wholesaler, strongly believes it was a trip that saved his life.
"I was one of the fortunate ones, just so lucky when I think about it," he said.
Chris initially thought he may have had an ulcer and was not concerned.
"My health was fine, I was a 40-something rugby player. I worked shifts and thought any tiredness was down to an unusual sleep pattern."
But after receiving a pinprick test at his local blood clinic in Ballymoney, nurses told Chris that he could not donate blood because his iron levels were unusually low and advised him to visit his local GP who then referred him for a colonoscopy.
Chris was then diagnosed with GIST, a rare cancer that occurs in the wall of the stomach and small intestine.
Doctors discovered the 9cm (3.5in) tumour in March 2013 and carried out a successful operation at the Royal Victoria Hospital in May.
"I tried to keep things as normal as I could at home, I have two young girls. Thankfully the cancer didn't spread.  I had surgery to remove my stomach. I was 16 stone (100kg) before surgery, now I'm 12 stone (79kg).
The Northern Ireland Blood Transfusion Service (NIBTS) has shared Chris' story on their Facebook page, describing it as a "story with a difference".
In a post, Chris said: "If it hadn't been for me giving blood, I never would have known about this until it would have been too late.
Now, Chris McIlmoyle is enjoying his life and has returned to work full-time. He has completed an abseil and bungee jump down Belfast City Hospital with his wife in aid of Friends of the Cancer Care Centre.
According to the Northern Ireland Blood Transfusion Service (NIBTS), some 6% of the eligible population donate blood in Northern Ireland.
A spokesperson said: "Most people aged between 17 and 65 years of age could give blood and save lives but unfortunately don't."
Writing on Facebook, Chris said: "Donating blood saves lives and in a different way than normal, it saved mine. Every blood test is like a mini MOT."
A spur of the moment decision to donate blood changed the life of 42-year-old County Antrim man Chris McIlmoyle.