It is unknown if physical activity and diet quality are associated with the risk of poor outcomes, such as mortality, among prefrail and frail older adults. This was a population-based cohort study among 1,487 prefrail and frail older-adults from the Third National Health and Nutrition Survey. Compared to participants who were sedentary (0 bouts of physical activity per week), those who were physically inactive (1–4 bouts of physical activity per week) were 24% less likely to die [HR: 0.76 (95% CI: 0.58–0.98)], and those who were physically active (≥5 bouts of physical activity per week) were 34% less likely to die [HR: 0.66 (95% CI: 0.51–0.86); Ptrend=0.002]. Compared to participants with poor diet quality, those with fair diet quality were 26% less likely to die [HR: 0.74 (95% CI: 0.52–0.98)], and those with good diet quality were 33% less likely to die [HR: 0.67 (95% CI: 0.55–1.00); Ptrend=0.050]. There was a synergistic interaction between physical activity and diet quality on the risk of mortality (Pinteraction=0.058). Participation in physical activity and consumption of a healthy diet is associated with a lower risk of mortality among prefrail and frail older adults.