Abnormal cognitive ageing, including dementia, poses serious challenges to health and social systems in ageing populations. As such, characterizing factors associated with abnormal cognitive ageing and developing needed preventive measures are of great importance. The ε4 allele of the Apolipoprotein E gene (
APOE4
) is a well-known genetic risk factor for late-onset Alzheimer’s disease.
APOE4
 carriers are also at elevated risk of cardiovascular diseases which are associated with increased risk of cognitive impairment. On the other hand,
APOE4
is known to be associated with reduced risk of multiple common types of cancer—a major age-related disease and leading cause of mortality. We conducted the first-ever study of
APOE4’s
opposing effects on cognitive decline and mortality using competing risk models considering two types of death—death with high-amounts versus low-amounts of autopsy-assessed Alzheimer’s neuropathology. We observed that 
APOE4
 was associated with decreased mortality risk in people who died with low amounts of Alzheimer’s-type neuropathology, but 
APOE4
 was associated with increased mortality risk in people who died with high amounts of Alzheimer’s-type neuropathology, a major risk factor of cognitive impairment. Possible preventive measures of abnormal cognitive ageing are also discussed.