This study investigated early posture development prospectively in infants at heightened (HR) vs. low risk (Low Risk; LR) for ASD. Fourteen HR infants diagnosed with ASD (HR-ASD), 17 HR infants with language delay (HR-LD), 29 HR infants with no diagnosis (HR-ND), and 25 LR infants were videotaped at home for 25 min during everyday activities and play at 6, 8, 10, 12, and 14 months. All postures were coded and the sustainment source was identified for supported postures. Relative to LR infants, HR-ASD infants and to a lesser extent HR-LD infants exhibited distinct postural trajectories that revealed slower development of more advanced postures. In addition, subtle differences in posture sustainment differentiated HR-ASD from HR-LD infants.