The purpose of this study was to assess the association between muscle thickness and echo intensity of the knee extensors and ankle plantarflexors with postural sway, mobility and physical function in older adults. Twenty-one older men and women (age; 69.9 ± 4.3 years) were assessed for postural sway (centre of pressure movement), mobility (i.e. Timed-Up and-Go-test [TUG]), physical function (i.e. 5 times sit-to-stand [STS]), and ultrasound derived measures of muscle thickness and echo intensity of the vastus lateralis (VL) and gastrocnemius medialis (GM). Significant inverse correlations were observed between VL and GM thickness with TUG (r = −0.432 to −0.492) and STS (r = −0.473 to −0.596). Significant positive correlations were observed between VL and GM echo intensity with TUG (r = 0.459 to 0.518) and STS (r = 0.481 to 0.635). Significant positive correlations were also detected between GM echo intensity and anteroposterior sway (r = 0.451 to 0.534). Two key findings emerged from the present experiment. First, this study provides novel evidence that ankle plantarflexor echo intensity, but not thickness, was associated with anteroposterior postural sway among older adults. Second, we provide new evidence that muscle thickness and echo intensity of the knee extensors and uniquely, the ankle plantarflexors, presented with similar magnitude associations with TUG and STS performance in older adults.