Background: Canada was the first country to develop legal mechanisms that allow for supported decision making, and little research has explored how decision making is supported in this context. This research aimed to understand how seven people with intellectual disabilities, living in two Canadian provinces, were supported with their decision making. Method: The research used constructivist grounded theory methodology, interviewing and observing the decision making of seven people with mild to severe intellectual disabilities and 25 decision supporters. Results: A common process of decision-making support was discovered, involving dynamic interaction between the person's will and preferences and supporters' responses. This interaction was influenced by five factors: the experiences and attributes the person and their supporter brought to the process; the quality of their relationship; the decision-making environment and the nature and consequences of the decision. Conclusion: The highly individualised and contextually dependent nature of decision-making support has implications for supported decision-making practice.