Background/Objectives: Sarcopenia is a public health concern linked to frailty. Accurate assessment is essential, yet standardization remains a challenge. This study assessed the agreement between Dual-energy X-ray Absorptiometry (DXA) and A-mode Ultrasound (US), evaluating their independent capacity to explain functional performance in community-dwelling older adults.  Subjects/Methods: This cross-sectional study included 136 adults (≥ 60 years). Body composition was measured via DXA (Appendicular Lean Soft Tissue Index, ALST/h²) and A-mode US (Anterior Thigh Muscle Thickness, MT). Functional performance was assessed using the Sit-to-Stand (STS) test (low performance: ≥15 seconds). Multiple linear regression models, adjusted for age, BMI, and sex, identified predictors of STS time.  Results: ALST/h² and anterior thigh MT were correlated with STS time (rho = 0.317, ρ=<0.001;rho=–0.293, ρ = 0.002) but weakly correlated with each other (r = 0.088, ρ = 0.310). The adjusted model explained 25.5% of STS variance (R² Adjusted = 0.255, p < 0.001). Age was the strongest independent predictor (β = 0.405,p < 0.001). The independent contribution of morphological measures was attenuated when age was included in the model. No significant interaction was found between age and ALST/h² (p = 0.465) or age and MT (p = 0.859).  Conclusion: Chronological age associates more strongly with functional performance than muscle quantity. Weak agreement between DXA and A-mode US suggests these methods are not interchangeable. While muscle mass and thickness explain little functionality (2–5%), morphology alone is insufficient to predict performance, likely due to age-related declines in muscle quality (e.g., myosteatosis) that A-mode US cannot distinguish.