Individuals high in narcissism excel when opportunities for personal glory are evident, and disappoint when no such opportunity exists. However, the mechanisms underlying these performance effects are unknown. Across two studies, we provide the first evidence that changes in effort explain narcissists’ performances. In Study 1 (n = 120), participants performed a dart-throwing task under high and low self-enhancement opportunity and self-rated their effort. In Study 2, we used an endurance task, again performed under low and high self-enhancement opportunity, but supplanted self-report measures of effort with psychophysiological measures. In both studies, narcissism had a significant positive indirect effect on performance via effort when self-enhancement was high but a negative indirect effect on performance when self-enhancement was low. Moreover, in Study 2 (n = 63), we tested an efficiency-based explanation of effort to examine whether individuals high in narcissism performed better under pressure because they “try harder” or because they “try smarter.” Results supported the “try harder” explanation. These data demonstrate that individuals high in narcissism excel when opportunity for success exists, thanks to their greater investment in the task.