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."

The UN children's fund, Unicef, says in a new report that the hopes of a generation across the Middle East and North Africa are being shattered.
It says that in Syria, Iraq, Yemen and Libya, almost 9,000 schools are not able to be used for education.
Unicef also documented scores of attacks on schools and teachers across the region.
Peter Salama, regional director for Unicef in the Middle East and North Africa, said: "The destructive impact of conflict is being felt by children right across the region.
"It's not just the physical damage being done to schools, but the despair felt by a generation of schoolchildren who see their hopes and futures shattered."
The 13.7 million who are out of education represents about 40% of the school-age children in Syria, Iraq, Yemen, Libya and Sudan and the UN fears that figure could top 50% in the coming months.
Unicef says that, in 2014, there were 214 attacks on schools in Syria, Iraq, Libya, the Palestinian territories, Sudan and Yemen.
One in four schools in Syria have shut since March 2011, affecting more than two million children, it says.
Unicef's Education Under Fire report added that "the killing, abduction and arbitrary arrest of students, teachers and education personnel have become commonplace" in the region.
Thousands of teachers have abandoned their posts in fear and children are often forced to work illegally to help their families.
The children can also become combatants at a young age.
Mr Salama said Unicef needed an additional $300m this year to try to improve access to education in the region.
A spur of the moment decision to donate blood changed the life of 42-year-old County Antrim man Chris McIlmoyle.
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The UN has said that conflicts in the Middle East are depriving more than 13 million children of an education.