Incomplete voluntary muscle activation may contribute to impaired muscle mechanical function and physical function in older adults. Exercise interventions have been shown to increase voluntary muscle activation, although the evidence is sparse for mobility-limited older adults, particularly in association with physical function. This study examined the effects of 12 weeks of power training on outcomes of voluntary muscle activation and gait speed in mobility-limited older adults from the Healthy Ageing Network of Competence (HANC) study. We included 37 older men and women with a usual gait speed of <0.9 m/s in the per-protocol analysis: n = 16 in the training group (TG: 12 weeks of progressive high-load power training, 2 sessions per week; age: 82.3 ± 1.3 years, 56% women) and n = 21 in the control group (CG: no interventions; age: 81.6 ± 1.1 years, 67% women). Knee extensor muscle thickness (ultrasonography), strength (isokinetic dynamometry), voluntary activation (interpolated twitch technique), and gait speed (2-min maximal walking test) were assessed at baseline and post-intervention. At baseline, TG and CG were comparable for all measures. Post-intervention, significant between-group changes (TG vs. CG; p < 0.05) were observed for voluntary muscle activation (+6.2%), muscle strength (+13.4 Nm), and gait speed (+0.12 m/s), whereas the between-group change in muscle thickness was non-significant (+0.08 cm). Improvements in voluntary muscle activation were associated with improvements in gait speed in TG (r = 0.67, p < 0.05). Importantly, voluntary muscle activation is improved in mobility-limited older adults following 12-weeks of progressive power training, and is associated with improved maximal gait speed. Incomplete voluntary muscle activation should be considered one of the key mechanisms influencing muscle mechanical function and gait speed in older adults.