Despite a history of conceptual contributions to reducing health inequalities by addressing the social determinants of health (SDH), Canadian governmental authorities have struggled to put these concepts into action. Ontario's—Canada's most populous province—public health scene shows a similar pattern. In statements and reports, governmental ministries, professional associations and local public health units (PHUs) recognize the importance of these issues, yet there has been varying implementation of these concepts into public health activity. The purpose of this study was to gain insight into the key features responsible for differences in SDH-related activities among local PHUs. We interviewed Medical Officers of Health (MOH) and key staff members from nine local PHUs in Ontario varying in SDH activity as to their understandings of the SDH, public health's role in addressing the SDH, and their units' SDH-related activities. We also reviewed their unit's documents and their organizational structures in relation to acting on the SDH. Three clusters of PHUs are identified based on their SDH-related activities: service-delivery-oriented; intersectoral and community-based; and public policy/public education-focused. The two key factors that differentiate PHUs are specific ideological commitments held by MOHs and staff and the organizational structures established to carry out SDH-related activities. The ideological commitments and the organizational structures of the most active PHUs showed congruence with frameworks adopted by national jurisdictions known for addressing health inequalities. These include a structural analysis of the SDH and a centralized organizational structure that coordinates SDH-related activities.