Metals industries producing large structures currently have a particular interest in hybrid laser welding processes, which possess advantages compared with conventional methods of welding. One major benefit is a reduction in deformation that enables the amount of postweld finishing work to be reduced. Assembly then also becomes simpler because of the greater accuracy that may be achieved. Larger joint tolerances may be accommodated compared with laser welding alone. By using appropriate filler metal, the weld metal composition may be controlled to meet metallurgical criteria. The hybrid CO2 laser–gas metal arc (GMA) welding process was investigated in this study; the aim being to clarify the effects of process gas composition on welding performance, weld cross section, quality, and mechanical properties, when welding carbon manganese steel. Helium, argon, and carbon dioxide were used in varying proportions as shielding gases for welding I-butt and T-butt joints. The composition of the shielding gas was found to affect welding performance, weld quality, and weld cross-section geometry. The best results were obtained by using argon, with a helium content of 40%–50% and a carbon dioxide content of 2%–5%; the exact composition depending on the groove edge quality. Hybrid laser welds of high quality may be produced in carbon manganese steel with a variety of shielding gas compositions, but to maximize productivity, reliability, and quality, while minimizing gas cost, the composition must be selected carefully.