Mechanistic reasoning involves analyzing the cause–effect relationships underlying a particular phenomenon. Science education recognizes the importance of supporting mechanistic reasoning development to prepare students for civic participation. However, less is known about whether and how non-experts apply mechanistic reasoning in civic contexts. We argue that understanding the nature of mechanistic reasoning used by laypersons in their functional everyday decisions could inform formal curriculum design to support students’ development of mechanistic explanations. We investigate patients who, facing gastrointestinal cancer or type 1 diabetes mellitus, are inherently compelled to make daily disease-related decisions. Our findings revealed that 33–35% of patients’ explanations incorporated mechanistic reasoning within the context of their own health condition which, albeit incomplete, had a significant impact on their decision-making self-efficacy. The findings extend beyond disease and health management, revealing the nature and depth of mechanistic reasoning as part of functional daily life issues, thereby establishing a basis for instructional interventions.