In this article, we report on a serious game development approach, characterized by combining theory-based design with an iterative development strategy guided by experimental test and evaluation. We describe two serious games that teach the mitigation of cognitive biases (human tendencies to commit systematic errors in thinking that lead to irrational judgments). Cognitive biases tend to be deeply ingrained and early attempts to reduce biases with training have met with little success. We address this training challenge using bias mitigation theory derived from the literature and an instructional framework to establish the educational content of each game. The mitigation effects of the games were measured through multiple experiment cycles, and multiple play-testing campaigns were conducted to inform instructional model and game design revisions. The final game versions achieved a medium-to-large training effect following a single play session.