This study captures life-long body mass index (BMI) trajectories across two related generations and estimates their associated mortality risks and population attributable deaths. We use prospective cohort data from the Framingham Heart Study (1948-2011) original (n = 4,576) and offspring cohorts (n = 3,753). Using latent trajectory models, we define seven BMI trajectories among original cohort and six among offspring cohort. People who are normal weight at age 31 and gradually move to overweight status in middle or later adulthood have the lowest mortality risk even compared to those who maintain normal weight status throughout adulthood, followed by overweight stable / upward, lower level of normal weight, overweight downward (original cohort only), class I obese upward and class II/III upward trajectories. Population attributable deaths associated with unhealthy weight trajectories have grown over generations because the prevalence has increased, offsetting the decline in trajectory-specific mortality risks.