They suspect marriage helps buffer against big heart disease risk factors, including cholesterol and high blood pressure.
The study looked at deaths from all causes, including heart disease.
Men and women in their 50s, 60s and 70s with high cholesterol were 16% more likely to be alive at the end of the 14-year ACALM study if they were married rather than single.
The same was true for diabetes and high blood pressure, with married people having a survival advantage.
The picture was less clear for people cohabiting, separated, divorced or widowed.
Also, the researchers did not test if the wedded people were in happy marriages.
They suspect having someone special in your life is what's important, rather than simply getting hitched.
Dr Carter said: "We need to unpick the underlying reasons a bit more, but it appears there's something about being married that is protective, not only in patients with heart disease but also those with heart disease risk factors.
"We're not saying that everyone should get married though.
"We need to replicate the positive effects of marriage and use friends, family and social support networks in the same way."
Dr Mike Knapton, of the British Heart Foundation, said: "The take-home message is that our social interactions, as well as medical risk factors such as high blood pressure, are important determinants of both our health and wellbeing.
"Whether you are married or not, if you have any of the main risk factors for heart disease, then you can call upon loved ones to help you to manage them."