Objective To determine the risk of mortality associated with and quantify the deaths attributable to combinations of body mass index (BMI) and waist circumference (WC). Methods This study included 41,439 participants. For the hazard ratio (HR) calculation, adiposity categories were defined as: BMIN/WCN, BMIN/WCO, BMIO/WCN, and BMIO/WCO (N = non-obese, O = obese). For the population attributable fraction analysis, obesity was classified as: (i) obese by BMI and/or WC; (ii) obese by BMI; and (iii) obese by WC. Mortality data was complete to the end of 2012. Results The prevalence of BMIN/WCN, BMIN/WCO, BMIO/WCN, and BMIO/WCO was 73%, 6%, 6%, and 15%, respectively. There was an increased risk of all-cause and cardiovascular disease (CVD) mortality in those with BMIN/WCO (HR (95% CI) 1.2 (1.2, 1.3) and 1.3 (1.1, 1.6)) and BMIO/WCO (1.3 (1.3, 1.4) and 1.7 (1.5, 1.9)) compared to those with BMIN/WCN. The estimated proportion of all-cause and CVD mortality attributable to obesity defined using WC or using BMI and/or WC was higher compared to obesity defined using BMI. Conclusions Current population obesity monitoring misses those with BMIN/WCO who are at increased risk of mortality. By targeting reductions in population WC, the potential exists to prevent more deaths in the population than if we continue to target reductions in BMI alone.