Background: Cancer-related fatigue and loss of physical functioning are distressing symptoms which negatively impact the quality of life of people with advanced cancer. Physical activity has been shown to have positive effects on these symptoms in early stage cancer, but previous research has demonstrated an incongruence between people with advanced cancer’s expressed interest and actual participation in a physical activity intervention. Aim: The aim of this two-phase, cross-sectional study was to gain an in-depth understanding of the experience of activity and quality of life in people with advanced cancer, using a classic grounded theory approach. Methods: Phase One involved participants wearing an activPAL™ activity monitor and filling out a daily record sheet for seven days duration; Phase Two involved face-to-face, semi-structured interviews using the daily record sheets and activity monitor outputs as qualitative probes. From an outpatient department of a tertiary cancer centre in Alberta, Canada, theoretical sampling was used to recruit a total of 15 people with advanced cancer and a median survival of 100 days. Data analysis employed classic grounded theory procedures, including core category emergence, constant comparison of indicators to theoretical saturation, and conceptual memoing. Findings: Maintaining responsibility emerged as the main concern of participants in this study, and downsizing to the critical threshold of responsibility accounted for how this concern was managed. The grounded theory explains how the critical threshold is influenced by conditions unique to people with advanced cancer, and how it is facilitated through activity and other mechanisms. Conclusion: Knowledge of this mid-level theory enables researchers and clinicians to understand activity as a mechanism through which the critical threshold of responsibility is managed, and to inform future behavioural interventions using a theoretical framework which is aligned with the advanced cancer experience.