Independent and small businesses often rely on underdog positioning strategies to gain market share against larger and more established companies. However, the effectiveness of these strategies remains unclear. The current study aims to investigate how different consumer personalities may influence their responses towards underdog positioning strategies. Two experimental studies with U.K. consumers (n = 349) show that the relationship between underdog status and positive attitudes is not as straightforward as previously believed. The research uses the lens of self-efficacy theory and found that underdog status positively correlates with perceived effort, consumer preference, and willingness to commit only among consumers with high trait agreeableness. In other words, although consumers generally acknowledge the efforts exerted by underdog providers, our study found that only agreeable consumers are more likely to reciprocate these efforts with increased positive attitudes. These findings contribute to the growing literature that examines the efficacy of the underdog effect. Subsequently, the findings have strong implications in ensuring the sustainability of small businesses by ensuring that marketing spending is optimized to target only the most effective consumer segments.