Schizophrenia and bipolar disorder broadly overlap in multiple areas involving clinical phenomenology, genetics, and neurobiology. Still, the investigation into specific elementary (sub-)processes of executive functioning may help to define clear points of distinction between these categorical diagnoses to validate the nosological dichotomy and, indirectly, to further elucidate their pathophysiological underpinnings. In the present behavioral study, we sought to separate common from diagnosis-specific deficits in a series of specific elementary sub-functions of executive processing in patients with schizophrenia and bipolar disorder. For our purpose, we administered a modern and multi-purpose neuropsychological task paradigm to equal-sized and matched groups of schizophrenia patients, patients with bipolar disorder, and healthy control subjects. First, schizophrenia patients compared to the bipolar group exhibited a more pronounced deficit in general measures of task performance comprising both response speed and accuracy. Additionally, bipolar patients showed increased advance task preparation, i.e., were better able to compensate for response speed deficits when longer preparation intervals were provided. Set-shifting, on the other hand, was impaired to a similar degree in both patient groups. Finally, schizophrenia patients exhibited a specific deficit in conflict processing (inhibitory control) and the shielding of task-relevant processing from distraction (i.e., attentional maintenance). The present investigation suggests that specific neuropsychological measures of elementary executive functions may represent important points of dissociation between schizophrenia and bipolar disorder, which may help to differentiate the pathophysiological underpinnings of these major psychiatric disorders. In this context, the present findings highlight the measures of inhibitory control and attentional maintenance as promising candidates.