The monotonic behavior of externally post-tensioned steel–concrete composite girders was numerically studied in this paper. A three-dimensional numerical model was developed and validated using experimental test results that were conducted by the authors and using an existing analytical model. A parametric study using this validated numerical model was performed to investigate the effects of various parameters on the monotonic performance of composite girders strengthened with external post-tensioned tendons. The parameters investigated include variations in the degree of shear connection, layout and diameter of shear connectors, the initial post-tensioning force, the depth of the steel beam, the eccentricity of the tendons, the compressive strength of concrete, and the shear capacity of the studs. The numerical model provided a better understanding of the effect of these parameters on the behavior of the strengthened beams. The results of the parametric study show that as the degree of shear connection decreased, the stud strains increased and the slippage between the concrete deck and steel beam increased. As the full degree of shear connection was reduced to 80%, 60%, and 40%, the stud strains were increased by 20%, 46%, and 94%, respectively. Also, as the shear connection degree decreases, its effect on the slippage behavior increases. As the degree of shear connection was reduced to 80%, 60%, and 40%, the slippage values were increased by 23%, 48%, and 102%, respectively. The study also shows that, for the same degree of shear connection, beams with one row of shear studs had up to 10% higher flexural capacity than beams with two rows of studs. A 100% increase in the stud diameter caused a 28% reduction in the ultimate load capacity and a 160% increase in the maximum slippage. The higher the post-tensioning force, the higher the ultimate load capacity and the lower the tensile strains in the steel beam. A 118% increase in the post-tensioning force resulted in a 9% increase in the ultimate load capacity and a 13% reduction in the tensile strains. Increasing the depth of the steel beam by 40% resulted in a reduction in the tensile strains at midspan by 20%. The lowest midspan tensile strains were obtained from the combination of increasing the depth of the steel beam and tendon eccentricity.