We investigated whether long-term hormone replacement therapy (HRT) is associated with mobility and lower limb muscle performance and composition in early postmenopausal women. A sample of fifteen 54-62-year-old monozygotic female twin pairs discordant for HRT was recruited from the Finnish Twin Cohort. Habitual (HWS) and maximal walking speeds (MWS) over 10 meters, thigh muscle composition, lower body muscle power assessed as vertical jumping height and maximal isometric hand grip and knee extension strengths were measured. Intra-pair differences (IPD%) with 95% confidence intervals were calculated. The mean duration of HRT use was 6.9 ± 4.1 years. MWS was mean 7% (0.9 to 13.1%; p=0.019) and muscle power 16% (-0.8 to 32.8%, 0.023) greater in the HRT users compared to their co-twins. Thigh muscle cross-sectional area tended to be larger (IPD%= 6%; 95% CI, -0.07 to 12.1%, p=0.065), relative muscle area greater (8%, 0.8 to 15.0%, 0.047), and relative fat area smaller (-5%, -11.3 to 1.2%, 0.047) in the HRT users compared to their sisters. There were no significant differences in maximal isometric strengths or HWS between the users and the non-users. Subgroup analyses revealed that estrogen-containing therapies (11 pairs) significantly decreased total body and thigh fat content, whereas tibolone (4 pairs) tended to increase muscle cross-sectional area. This study showed that long-term HRT was associated with better mobility, greater lower body muscle power and favorable body and muscle composition among 54-62-year-old women. The results indicate that HRT is a potential agent in preventing muscle weakness and mobility limitation in older women.