Aims: The Montreal Cognitive Assessment (MoCA) has gained recognition for its validity in detecting cognitive impairment in several clinical populations. For serial assessments, alternate forms are needed to overcome possible practice effects. Our objective was to investigate the reliability of two German MoCA alternate forms for longitudinal assessment applications. Methods: The original and one of two alternate forms of the MoCA were administered within a 60-min interval of a clinical interview in a counterbalanced order to 100 healthy elderly controls, 30 patients with mild cognitive impairment (MCI) and 30 patients with Alzheimer’s disease (AD). The diagnosis of the majority of patients was supported by in vivo AD pathology biomarkers. Results: There was a strong correlation between the alternate forms and the original MoCA in all groups, but particularly in the clinical samples. Total mean scores did not differ significantly between the MoCA versions, even taking into account the presentation order. As in previous studies, age and education influenced performance in the MoCA. The same pattern of group differences (controls > MCI > AD) was observed for each of the versions. Conclusion: All three forms can be reliably and interchangeably used in serial cognitive assessment, confirming the MoCA’s applicability in research and clinical longitudinal approaches.