Introduction: Mobility is an important indicator of individual functioning. Patient’s balance and mobility are the key issues for assessment of ability to perform motor activities. De Morton mobility index (DEMMI) is a standardized measurement tool for mobility assessment and includes 15 items. Purpose: to translate DEMMI into Slovenian language and determine metric properties of the translation in patients with musculoskeletal impairments participating in rehabilitation. Methods: nine participants with musculoskeletal impairments in rehabilitation, aged 78 years on average (SD 6 years; range 71–85 years) were included in the inter-rater reliability pilot study, which was part of the cross-cultural adaptation process. In the study of validity, responsiveness, minimal clinical important difference, floor and ceiling effects, 30 patients with musculoskeletal impairments with or without impairments of peripheral nerves, aged 54 years on average (SD 18 years; range 22–84 years), were assessed with the translation of DEMMI, functional ambulation classification, 10 meter walk test, 6 minute walk test, Berg balance scale and motor part of functional independence measure. Assessment was performed at admission to rehabilitation, after two and four weeks of treatment. Spearman’s correlation coefficient calculation (rho) between DEMMI and other measurement tools was used for evaluating validity. Differences in the scores of each measurement tool were established with paired t test. For the known groups validity independent t test was calculated. The effect size was used to calculate measurement responsiveness. Minimal clinical important difference was calculated using the distribution based method, and floor and ceiling effects with percentage of participants who scored the lowest or highest scale score. Results: The inter-rater reliability was excellent (τ = 0.82–0.97). Concurrent validity for DEMMI was good to very good (rho = 0.55–0.92). Participants who were not able to walk had significant lower DEMMI scores (average 26; SD 8) than participant who walked with walker (average 33; SD 4) or crutches (average 44; SD 8). DEMMI was highly responsive to change mobility after two weeks (effect size = 1.15), and even higher after four weeks (effect size = 1.97). Minimal clinical important difference was 5 points, no floor and ceiling effects were identified. Conclusions: Slovenian translation of DEMMI is comprehensible. It is valid and sensitive measurement tool, which differentiates mobility level of known-groups participants, and has no floor or ceiling effects in patients with musculoskeletal impairments in rehabilitation, regardless of age. Despite different causes and levels of mobility decline in patients’ assessment with DEMMI showed improvement of mobility after two and four weeks of rehabilitation, therefore we recommended it for use in physiotherapy. A translation of DEMMI will be published for use in Slovenia.