Incidental memory can be defined as the ability to acquire information unintentionally. The present study investigated incidental memory performance in amnestic mild cognitive impairment (aMCI) and mild Alzheimer’s disease (AD) patients; additionally, hippocampal atrophy between groupswas examined. Twenty-nine aMCI patients (14 with hippocampal atrophy, measured by the Medial Temporal Lobe Atrophy scale), 15 mild AD patients, and 20 cognitively intact individuals underwent a detailed medical and neuropsychological assessment examining intentional memory, using the Hopkins Verbal Learning Test-Revised and the Brief Visuospatial Memory Test. Participants first took part in a driving simulator experiment, followed by an unexpected incidental memory questionnaire referring to elements related to the driving simulation. The mild AD group performed worse than the aMCI group and the control group both in incidental and intentional memory tasks, whereas the aMCI group differed significantly from the control group only in the intentional memory tasks. The incidental recognition memory task was the only measure that differed between aMCI patients with and without hippocampal atrophy. Moreover, incidental memory tasks were the only measures that correlated significantly with both left and right hippocampal atrophy. The current findings indicate that incidental memory testing may provide potentially useful information for detecting aMCI patients with greater hippocampal atrophy, who may be considered at higher risk of developing dementia due to AD.