The use of constitutive models in thermomechanical finite element modelling of refractory linings requires knowing the temperature-dependent material parameters. The mechanical testing of carbon-containing refractory materials at elevated temperatures necessitates the protection of samples from oxidation. Therefore, the test concept of the modified shear test (MST) was further developed and a setup was designed to protect the carbon-containing materials from oxidation. A carbon-containing magnesia refractory (MgO–C), which is usually applied in secondary metallurgy for steel ladle refractory linings, was selected as the material of interest. The setup allows the determination of cohesion and friction angle of MgO–C refractories under reducing conditions at temperatures up to 1500 °C. The procedure allows a material parameters determination from uniaxial loading. While coked and as-delivered samples showed different behaviours, a significantly higher cohesion was noted in the as-delivered material. The results showed that the cohesion is highly temperature-dependent, whereas the friction angle remains nearly unaffected.