Research in the context of the mood-behavior-model (Gendolla in Rev Gen Psychol 4:348–408, 2000) has shown that moods can have an impact on effort mobilization due to congruency effects on demand appraisals. However, the mood research literature suggests that mood may also influence effort mobilization by its impact on appraisals of the instrumentality of success. In a single factor (mood valence: negative vs. neutral vs. positive) between-persons design, participants performed a memory task under conditions of unclear task difficulty. By successfully performing the task, participants could earn the chance to win a monetary reward. As predicted for tasks with unclear difficulty, effort mobilization—assessed as cardiovascular reactivity—increased from negative to positive mood. This effect was mediated by the subjective probability of winning the monetary reward for successful performance. These results demonstrate for the first time that mood can influence effort mobilization via the estimated instrumentality of success.