Many employees self-label as allies, yet few engage in concrete allyship action. We propose that one critical barrier is impostorism – possessing allyship competencies yet feeling like a fraud – which may stifle action and undermine individual mental health and workplace functioning. Latent profile analyses tested whether this unique confluence of competencies and feelings of fraudulence were present with respect to the ally role. Results from a preregistered study with two representative samples of working adults in Michigan, US (N = 778) and Canada (N = 973), identified four profiles: (1) Competent Ally Impostors (high allyship competencies, high impostorism), (2) Confident Allies (high allyship competencies, low impostorism), (3) Average Allies (average allyship competencies, moderate impostorism), and (4) Disengaged Individuals (low allyship competencies, low impostorism). As hypothesized, we found that Competent Ally Impostors had significantly higher levels of depressive symptoms and anxiety than Confident Allies, suggesting that this subgroup of skilled individuals may nevertheless be impeded by impostorism, and their mental health may suffer as a result. Unlike our hypotheses, Disengaged Individuals and Average Allies reported significantly less psychological safety, work efficacy, and job satisfaction than Confident Allies and Competent Ally impostors suggesting that allyship competencies and not impostorism may drive employees’ levels of psychological safety, work efficacy, and job satisfaction at work. Our findings point to the outcomes associated with allyship impostorism and other subgroups of allyship functioning, which in turn sheds light on how we can better target research and practice to boost allyship.