Steel–concrete composite beams are commonly employed in civil constructions such as multi-storey building floors and long-span bridges. In case of significant span to depth ratios, excessive deflections and noticeable crack widths at the concrete slab may occur at the serviceability stage. In this context, internal or external pre-stressed tendons can be used as a remediation. In the latter case, significant effects of geometric non-linearity may arise due to the strain incompatibility between the external tendon and the composite member, resulting in a variable tendon eccentricity during deformation. In this paper, second-order effects are evaluated in eight pre-stressed steel–concrete composite beams for which experimental results are available. Hence, a three-dimensional finite element model using a Lagrangian description is proposed. The external tendons are represented by one-dimensional catenary elements, whereas the reinforced concrete slab and steel profile are modeled by shells finite elements. Slipping at the slab-beam interface and tendon-deviator locations are also included in the analysis. For the studied externally pre-stressed beams, the introduction of nonlinear geometric behavior yields lower collapses loads in relation to its linear counterpart, varying this difference between 6.4 and 9.1%.