Objective: To examine the association between the de Morton Mobility Index (DEMMI) score on admission in geriatric patients and readmission and mortality within 30, 180, and 365 days after discharge, and discharge to a post-acute care facility. Methods: A nationwide register-based cohort study including 23,941 geriatric in-patients aged ≥65 years admitted to a geriatric ward between 2014 and 2017 and included in the Danish National Database for Geriatrics. The DEMMI score was categorized into four subcategories: very low mobility (DEMMI=0–24), low mobility (DEMMI=27–39), moderately reduced mobility (DEMMI=41–57), and independent mobility (DEMMI=62–100). Patients were followed 30, 180 and 365 days after discharge for readmission and mortality. Their risk of being discharged to a post-acute care facility was examined. Adjusted hazard ratios (HRs) and odds ratios (ORs) with 95 % confidence intervals (CIs) were calculated. Results: HRs for readmission within 30-days were 1.36 (1.24–1.48) for very low mobility, 1.30 (1.20–1.42) for low mobility and 1.17 (1.08–1.28) for moderately reduced compared with independent mobility. Similar results were seen for readmission within 180- and 365-days. For mortality, HR for 30-day mortality ranged from1.93 and 5.66, 180-day mortality between 1.62 and 3.19, and 365-day mortality between 1.54 and 2.81 compared with patients with independent mobility. OR for discharge to a post-acute care facility was 8.76 (7.29–10.53) for lowest compared with the highest DEMMI mobility subcategory. Conclusion: In geriatric in-patients, lower DEMMI scores on hospital admission are associated with increased rates of discharge to a post-acute care facility, and for readmission, and mortality within one year.