(1) Background: Markerless motion capture may enable scalable biomechanical assessment outside laboratory environments, yet its applicability to tasks involving elevated foot contacts and rapid loading transitions remains insufficiently characterized. This feasibility study evaluated OpenCap, an open-source smartphone-based markerless system, for estimating lower-limb kinematics and ground reaction forces during stair ascent, stair descent, squat, sit-to-stand, and forward lunge. (2) Methods: One healthy adult completed three repetitions of each task while data were acquired simultaneously with three smartphones, an eight-camera Vicon system, and two force plates. OpenCap/OpenSim outputs were adapted for stair and lunge kinetics and compared with marker-based kinematics and force-plate measurements using root mean square error, Pearson correlation, and cosine similarity. (3) Results Agreement was strongest for sagittal-plane lower-limb kinematics, particularly knee flexion-extension (RMSE: 1.4-10.5 deg; Pearson: 0.94-0.999). Larger discrepancies occurred for hip internal-external rotation, pelvis motion during sit-to-stand, and ankle flexion-extension. Resultant GRF waveforms showed high similarity to force-plate data (cosine similarity >=0.988; RMSE: 0.069-0.167 body weight). Stair descent produced the largest deviations and variability, particularly for single-limb forces. (4) Conclusions These findings support the technical feasibility of smartphone-based markerless functional biomechanics while highlighting the need for improved multiplanar pose estimation, contact modeling, and larger validation studies.