Background: Few studies have investigated the impact of body mass index (BMI) and physical activity (PA) on mortality among colorectal cancer (CRC) patients and the results are inconsistent. We aimed to examine the impact of these lifestyle factors on all-cause and disease-specific mortality. Methods: Population-based longitudinal study followed 1,825 patients diagnosed with stages I to III primary CRC during 2003 to 2004 in Queensland, Australia for 5 years. Sociodemographics and clinical characteristics were obtained via questionnaires and medical records. Results: Participants with some level of PA following diagnosis had 25% to 28% lower risk of all-cause mortality within 5 years of diagnosis than sedentary participants [insufficiently active: HR = 0.72, 95% CI = 0.57-0.91; sufficiently active: HR = 0.75 (0.60-0.94)]; however, the differential for CRC-specific mortality was not significant. Increases in PA from five to 12 months postdiagnosis was associated with reduced CRC-specific mortality by 32% to 36% (increase ≤ 2 hour per week: HR = 0.68 (0.48-0.97); increase > 2 hour per week: HR = 0.64 (0.44-0.93) and 31% for all-cause mortality (increase >2 hour per week: HR = 0.69 (0.50-0.94). Compared with participants with healthy BMI, significant higher mortality risk was observed in underweight patients (all-cause: HR = 2.29 (1.47-3.59); CRC: HR = 1.74 (1.00-3.04), although lower risk in overweight (all-cause: HR = 0.75 (0.61-0.94); CRC: HR = 0.75 (0.59-0.97) and no difference in obese. Excessive weight loss was associated with increased mortality risk by three-fold but no difference in those who gained weight. Conclusions: Protective effects of being physically active and increasing that activity underlines the importance of interventions to increase activity levels among people being diagnosed with CRC. Impact: Increased mortality risks associated with being underweight or having weight loss over time is an important indicator for which clinicians, patients, and support personnel can monitor.