We have previously shown that the minicolumnar spacing of neurons in the cerebral cortex relates to cognitive ability, and that minicolumn thinning occurs in old age. The present study examines further the relationship between cognitive ability and cortical fine structure (minicolumn organization and neuropathology) in the dorsolateral prefrontal cortex (dlPFC) and the parahippocampal gyrus (PHG) in mild cognitive impairment (MCI) and Alzheimer's disease (AD). Premortem neuropsychological scores were related to postmortem microanatomy in 58 adults (20 normal controls, 18 MCI, and 20 confirmed AD patients). We found a correspondence between minicolumn thinning in the dlPFC and IQ decline in dementia. In mild impairment, IQ remained stable, as did dlPFC minicolumn width and dlPFC plaque load. IQ only declined as dlPFC minicolumn thinning occurred and dlPFC plaque load increased in more severe dementia. By contrast, plaque load increased and minicolumns became steadily thinner in the PHG, where minicolumn width correlated with declining mini-mental state examination score across both MCI and severe dementia. By including a further 14 younger control subjects, we found that in normal healthy aging, minicolumn width decreased in the dlPFC, whereas PHG minicolumn width did not change. AD patients in our dataset with higher IQ were older at time of death and had less pathology, which supports a neural basis for the cognitive reserve hypothesis.