Objectives: Androgen-deprivation therapy (ADT) for prostate cancer (PCa) is associated with reduced physical function and quality of life (QoL). We investigated the impact of a structured lifestyle intervention on the promotion of physical activity (PA) and reduction of sedentary behavior (SB), and its effect on QoL in men on ADT. Methods: Patients with advanced PCa on long-term ADT were randomized to the intervention (N = 11) or a control arm (N = 10) between February 2018 and May 2019. The intervention group received a structured lifestyle intervention including motivational text messages for 8 weeks (maintenance visit at week 12). At each visit, self-report measures and accelerometer data were used to assess PA and SB, and questionnaires were used to measure QoL, life satisfaction, anxiety, and depression. Results: Significantly greater improvements in QoL and depression compared to baseline were reported in the intervention group compared to the control group. In addition, the intervention group also showed a significantly greater increase in self-reported light, as well as moderate-to-vigorous PA, and reduction in self-reported SB. Conclusions: Given its inherent advantage in improving QoL and reducing depression, a lifestyle intervention program should be offered to patients on ADT.