This exploratory study examined the moderating effects of personality traits on cognitive function following a cognitively stimulating individualized activity intervention delivered to individuals at high risk for cognitive decline: those with delirium superimposed on dementia. Data were taken from an ongoing randomized clinical trial with the addition of a personality measure. The results for 71 participants randomized to intervention or control groups are reported. Significant moderating effects of personality traits were found such that participants with higher agreeableness were more likely to have improved delayed recall and those with lower extraversion were more likely to have improved executive function, as a result of the intervention. Lower openness, higher agreeableness, and lower conscientiousness were associated with greater engagement in the intervention. A cognitive stimulation intervention for older adults at high risk for further cognitive decline may be differentially effective based on certain personality traits.