This study investigates how managerial personality traits influence firms’ responsiveness to market information in capital investment decisions. Using a sample of U.S. listed firms from 2007–2018, we proxy CEO narcissism through signature size, a behavioral indicator widely associated with self-orientation and overconfidence. We document that firms led by more narcissistic CEOs exhibit weaker investment–Q sensitivity, suggesting that these executives place less weight on market-based signals when allocating capital. Importantly, we show that board composition and governance quality materially condition this relationship. Greater female representation on the board mitigates the attenuated investment response associated with CEO narcissism, consistent with enhanced monitoring and more balanced decision-making. Complementary analyses further indicate that stronger governance structures constrain the extent to which CEO narcissistic tendencies distort investment behavior. Our findings are robust to alternative specifications, endogeneity concerns, and estimation techniques. Overall, the study contributes to the literature on behavioral corporate finance by demonstrating that investment efficiency reflects an interaction between executive personality and board-level governance mechanisms, rather than managerial traits in isolation.