Objective We aimed to identify factors associated with all-cause mortality in adults with incident asthma. Design and setting Cross-sectional cohort study, in the metropolitan areas of Copenhagen and Aarhus, Denmark. Participants Adults aged 50-64 years enrolled in the Danish Diet, Cancer, and Health cohort were followed up from baseline (1993-1997) in the National Patients Registry for first-time admissions for asthma and vital status. We defined incident asthma as at least one first-time hospital admission with asthma as the primary registered diagnosis between baseline and end of follow-up (2013) in participants without previously known asthma. Among the cohort comprising 57 053 individuals, we identified 785 adults (aged 50-64) with incident asthma, of whom 76 died during follow-up. Primary and secondary outcome measures Baseline reported socioeconomic and lifestyle traits, and comorbidities associated with all-cause mortality. Results Self-reported leisure-time physical activity was associated with a substantial reduction in risk with an HR of 0.53 (95% CI 0.33 to 0.85). Being male, single and having a diagnosis of hypertension or diabetes were associated with an increased risk of all-cause mortality with an HR of 1.83 (95% CI 1.14 to 2.38), 2.16 (95% CI 2.06 to 4.40), 2.47 (95% CI 1.54 to 3.95) and of 2.42 (95% CI 0.96 to 6.11), respectively. Conclusions This long-term study of adults with hospital contacts for incident asthma revealed that self-reported leisure-time physical activity is associated with an approximately 50% reduction in all-cause mortality. In contrast, both hypertension and diabetes were associated with a higher risk of mortality.