A team from the Indiana University School of Medicine found that even mild cognitive impairment can reduce life expectancy.
They argue that early signs of decline should be prioritised in primary care in light of the significant risk to health.
Co-author Dr Greg Sachs said: 'We found that even mild cognitive impairment, as determined by a simple screening tool in a primary care physician's office, has a strong impact on how long individuals survive on the same order as other chronic diseases.'
The severity of the risk means clinicians should examine and treat the causes of mild mental impairment, such as delirium or adverse drug effects, to reduce the threat to health, the researchers said.
Studies have shown that patients with Alzheimer's disease have reduced life expectancy but few have examined risks among typical primary care populations.
The team led by Dr Christopher Callahan used records of 3,957 patients aged 60 and over who visited a health centre in Indiana between 1991-93.
Patients were screened for cognitive impairment using a 10-question survey and scored as having either no, mild or moderate to severe impairment.
By the end of 2006, 68% of patients with mild impairment had died compared with 57% of those with no mental decline at the start of the study. Nearly 80% of those with moderate to severe impairment had died.
Moderate to severe impairment had a 'striking effect' on life expectancy similar to the effect of diabetes or chronic heart failure, the researchers said. They concluded that the increased risk should be considered when making decisions on advance care planning, particularly in cases of severe impairment.