Purpose: This nationwide study assessed associations between comorbidity and mortality after hip fracture in young and middle-aged patients. Methods: Data on 19,682 patients aged 18 to 65 years were extracted from Danish registries out of 154,047 patients who experienced a hip fracture between 1996 and 2012. Mortality and comorbidity were assessed using information on vital status, hospital admissions, and prescriptions. Results: Of the 19,682 patients 17,722 (90.0%) were middle-aged (40–65 years) and 1960 (10.0%) were young (18–39 years). The 30-day mortality rates were 3.2% (n = 570) and 1.6% (n = 32), respectively. Indicators of multi-trauma (hazard ratio (HR), 3.5 95% confidence interval (CI) [1.6–7.8], n = 2056) and having diabetes (HR, 4.4 [1.2–11.3], n = 59) and heart disease (HR, 4.4[1.3–14.8], n = 57) increased 30-day mortality in the young patients, while having cancer (HR, 5.0 [4.2–5.9], n = 1958) increased 30-day mortality in the middle-aged patients. Conclusion: Heart disease and diabetes were associated with high mortality in the young patients while having cancer was associated with high mortality in the middle-aged patients.