Research tells us that leaders who cultivate a shared sense of social identity within their teams foster cohesion, collaboration, and productivity. I employed a randomised field experiment design, to examine how leader trait altruism and self-focused emotion regulation interact to influence identity leadership performance. Drawing on the 5R identity leadership framework, I randomly assigned 339 managers either to an identity leadership training group or a control group. I tasked participants with addressing a fictional workplace scenario requiring inter-departmental cooperation, and their responses were assessed for identity leadership performance. Results indicated that participants high in altruism and low in self-focused emotion regulation displayed high identity leadership performance, compared with those low in altruism and high in self-focused emotion regulation. Results showed that high self-focused emotion regulation inhibited the expression of empathy and prosocial communication necessary for effective leadership, thus limiting the positive impact of trait altruism. These findings highlight the hitherto unknown role of self-focused emotion in regulating the relationship between trait altruism and identity leadership. My research contributes to leadership theory by integrating trait-based and social identity perspectives, offering a nuanced understanding of how personality traits and emotional dynamics influence leadership outcomes. Practically, the study underscores the value of leadership training programs, such as the abridged 5R framework, which develop prosocial behaviours while mitigating the risks of emotional over-regulation.