We examine California Assembly Bill No. 979 (AB 979), the first law mandating racial, ethnic, and other forms of diversity on corporate boards. Conventional t-tests show that stock returns around the enactment of the law are negative, economically large, and statistically significant; returns are more negative for smaller firms and firms with no diverse directors, suggesting higher compliance costs. However, statistical significance disappears after controlling for event date cross-correlation, when comparing returns on event dates to the pre-event distribution of returns, and in multivariate regressions. At least 90 % of firms comply with the first stage of AB 979 and the qualifications of mandated directors are largely similar to those of benchmarks, suggesting compliance costs are low. We also find no evidence that compliance affects firm operating performance. Overall, our results suggest that the diversity mandate has statistically and economically small effects relative to typical variation in stock returns and firm outcomes, which highlights the importance of considering appropriate counterfactuals and examining multiple dimensions when analyzing the impact of such mandates.