We mobilize theories of corporate governance and the theory of the social psychology of decision-making small groups to understand the operational process of the public hospital supervisory board. More precisely, we empirically test the mediation relationship of the decision-making process (effort norms, use of knowledge and skills, and conflict cognitive) between its structure (size, the composition, and diversity) and the performance of its roles (strategy, control, and service). A total of 320 questionnaires coming from members of the French public hospital supervisory board were collected. The aggregation of these individual answers generates a sample of 159 public hospital supervisory boards. The results of the tests of the assumptions of the research model confirm the fact that the structure of the supervisory board does not influence the performance of its roles. However, supervisory board effort norms positively affect the performance of its roles positively. Only effort norms and the use of knowledge and the skills partially play the role of mediator between the supervisory board structure and the performance of its roles. Practical and theoretical implications are exposed in the discussion.