Diet and exercise are important for the wellbeing of people with cancer and their family caregivers. Unfortunately, little is known about their behaviors over time or factors that may influence their engagement in these behaviors. This exploratory study examined the influence of chronic conditions, symptom distress, and perceived social support on exercise and diet behaviors of patients with advanced cancer and their caregivers using the actor-partner interdependence mediation model (APIMeM) and interdependence theory as guiding frameworks. This secondary analysis uses self-report data from a large RCT (N = 484 patient-caregiver dyads) at three time points: baseline data was collected within three months of the diagnosis, at 3 months post-baseline, and 6 months post-baseline. A number of actor effects were observed: patient and caregiver prior exercise and diet were significant predictors of their own future exercise and diet behaviors; more patient-reported social support was associated with less patient exercise; more patient symptom distress was associated with poorer patient diet; and, more caregiver-reported social support was associated with more caregiver exercise and better caregiver diet. Partner effects were also observed: more patient exercise was positively associated with more caregiver exercise; more patient comorbidities were associated with better caregiver diet; more caregiver-reported social support was associated with better patient diet; and, more patient-reported social support was associated with better caregiver diet. Despite the challenges of advanced cancer and caregiving, past exercise and diet behavior remained a significant predictor of future behavior. Other health problems and perceptions of social support within the dyad may exert a positive or negative influence on patient/caregiver diet and exercise.