Background: Determinants of adherence to lifestyle regimens are ill understood. Attendance to intervention sessions is crucial for patients to acquire knowledge and skills regarding the core elements of an intervention. Therefore, we explored demographic, health-related, and social determinants of high and low attendance to diet and exercise sessions among overweight and obese patients with knee osteoarthritis (≥ 60 years; N = 206). Methods: The Arthritis, Diet, and Activity Promotion Trial was an 18-month randomized controlled trial on the effectiveness of dietary weight loss and exercise interventions. We conducted chi-square and t-tests, and logistic regression analyses on categories of short- and long-term attendance to intervention sessions. Results: Over the 18-month duration of the study, 60.7% (± 28.5) of diet sessions, and 53.2% (± 29.0) of exercise sessions were attended. Not being married, low social participation, and single intervention randomization predicted high attendance to diet sessions during months 1-4. Exercising at home, and single intervention randomization predicted high attendance to exercise sessions during months 5-18. High attendance to sessions early in the intervention was a significant determinant of high session attendance thereafter. Conclusions: Offering people a choice where to exercise, and stimulating early intervention session attendance can be effective in improving long-term attendance to both interventions. Several determinants we found may be amenable to change to enhance intervention adherence of future randomized controlled trials involving dietary weight loss and/or physical exercise.