Skeletal muscle atrophy and weakness resulting from loss of motor neuron innervation are shared hallmarks of amyotrophic lateral sclerosis (ALS) and age-associated sarcopenia. However, muscle wasting in ALS is rapid and severe, whereas sarcopenia progresses gradually over decades, peaking at an advanced age. The molecular mechanisms underlying muscle wasting in these conditions remain incompletely defined. Here, we used the SOD1G93A ALS mouse model and wild-type mice across the lifespan to generate an integrated single-nucleus transcriptomic atlas of skeletal muscle, mapping changes associated with ALS and aging. This atlas is complemented by structural and functional analyses. Our results identify transcriptionally distinct condition-specific myonuclear states, including disease-responsive programs within synaptic myonuclei at the neuromuscular junction, that are shared or distinct between the two conditions. Notably, at early stages, muscle from ALS mice exhibits molecular hallmarks of aged muscle at the myofiber level, indicating premature aging and reinforcing aging as a risk factor in ALS. At advanced disease stages and in old age, myonuclear subpopulations follow similar degenerative pathways in fast glycolytic fibers, consistent with their loss in both conditions. However, a distinct ALS-enriched myonuclei subpopulation displays uniquely regulated genes not observed in aged muscle, potentially underlying the more rapid and severe pathology observed in ALS. This atlas maps shared and divergent mechanisms of muscle degeneration in ALS and sarcopenia and provides a resource for identifying therapeutic targets tailored to each condition.