Large prospective studies of Alzheimer’s disease (AD) have sought to understand the pathological evolution of AD and factors that may influence the rate of disease progression. Estimates of rates of cognitive change are available for 12 or 24 months, but not for shorter time frames (e.g., 3 or 6 months). Most clinical drug trials seeking to reduce or modify AD symptoms have been conducted over 12- or 24-week periods. As such, we aimed to characterize the performance of a group of healthy older adults, adults with amnestic mild cognitive impairment (aMCI), and adults with AD on the CogState battery of tests over short test-retest intervals. This study recruited 105 healthy older adults, 48 adults with aMCI, and 42 adults with AD from the Australian Imaging, Biomarkers, and Lifestyle study and administered the CogState battery monthly over 3 months. The CogState battery of tests showed high test-retest reliability and stability in all clinical groups when participants were assessed over 3 months. When considered at baseline, the CogState battery of tests was able to detect AD-related cognitive impairment. The data provide important estimates of the reliability, stability, and variability of each cognitive test in healthy older adults, adults with aMCI, and adults with AD. This may potentially be used to inform future estimates of cognitive change in clinical trials.