His scholarship and his interest in questions of the day led him into affiliations with many of the learned societies. Nor were these affiliations perfunctory only. He regularly attended the society meetings, wrote papers for them, and rose to the highest places in their councils. He was president of the American Social Science Association (1897), International Law Association (1899), American Historical Association (1905), Political Science Association (1910), American Society for the Judicial Settlement of International Disputes (1911), Connecticut Academy of Arts and Sciences, Connecticut Society of the Archeological Institute of America (1914). He was vice-president of the Archeological Institute of America (1898) and of the social and economic science section of the American Association for the Advancement of Science (1903). He was an associate of the Institute of International Law. He was elected a Fellow of the American Academy of Arts and Sciences in 1912. He was a member also of the National Institute of Arts and Sciences, American Philosophical Society, elected a member of the American Antiquarian Society in 1893, and a corresponding member of the Massachusetts Historical Society, Colonial Society of Massachusetts, and L'Institut de Droit Comparé. Baldwin was among the incorporators of Connecticut College at its founding in 1911, and he served on its board of trustees through 1924.