Aim To assess the association between socioeconomic factors and mortality in Danish children diagnosed with different types of severe chronic disease, including cancer. Methods National cohort study 1994-2020 including Danish children with chronic disease. Inclusion was based on diagnoses in The National Patient Register, socioeconomic information was obtained from Statistics Denmark and mortality was ascertained from the Cause of Death Register. Hazard ratios (HR) with 95% confidence intervals (CIs) were based on Cox regression. The factors were combined in one common risk score and the association with disease-specific mortality was analysed overall and by ethnicity status. Results Overall, non-Danish ethnicity (HR = 1.96 (95% CI 1.69-2.28)) was associated with all-cause mortality in 128 129 children (69 435 male and 58 694 female) with chronic disease. Median age at first diagnosis was 1.42 years (range 0-18 years). Low family income was associated with mortality regardless of ethnicity status, and young maternal age was also a notable risk factor across ethnicities. The socioeconomic association was more pronounced in children with cancer. Conclusion In the high-income setting of Denmark, ethnicity and differences in socioeconomic background were associated with child mortality even among children with severe chronic disease. The pattern was more pronounced in paediatric cancer patients.