This study presents an experimental investigation on composite beams (CB) to explore the effectiveness of adding external post-tensioning (EPT) force, as a strengthening technique, on the monotonic flexural performance of composite samples. Two experiments on CB were conducted under three-point loading. High-strength steel strands post-tensioned to 46% of the ultimate strength were used to apply the EPT. Moreover, a finite element (FE) model is developed to present numerical investigations on the monotonic and fatigue behaviors of the CB with and without EPT by considering the slip between the concrete slabs and steel beams as well as the fatigue deterioration in the concrete slabs. A FE parametric study is conducted using the proposed FE model to investigate the effect of steel beam grade, tendon eccentricity, span-to-depth ratio, and fatigue loading. The optimum span-to-depth ratio to obtain the maximum contribution of the EPT force in improving the CB capacities is 9.0.