The creep behaviour of two ex-service type 316H austenitic stainless steel casts with different Mn contents and austenite grain size, that were pre-carburized for either 8000 h at 550°C or 3000 h at 600°C in simulated CO2-rich gas environment of UK Advanced Gas-cooled Reactors, was evaluated by uniaxial creep testing at 550°C in air. The low Mn (0.98 wt.%) steel grade with an average austenite grain size of 142±110 µm presents a higher creep deformation rate and creep ductility compared to the high Mn (1.52 wt.%) steel cast with an austenite grain size of 81±67µm. Below an applied stress of 318 MPa, the presence of the sub-surface carburized layer reduces the minimum creep rate, likely due to the significantly modified material properties of the carburized layer and the presence of local compressive stresses. The stress exponent determined for the high Mn steel carburized at 600°C (n=9.6), and its reference non-carburized microstructure (n=9.8), identifies the dominant creep mechanism to be dislocation climb & glide. However, crack evolution through M23C6-decorated grain boundaries within the carburized layer was observed in all the tested material's conditions at creep strains of 0.50%. The average crack length increases with creep strain and attains values close to the carburized layer thickness at creep strains of ε≥1.00%. The prevalence and size of cracking increases with the local hardness carburized layer depth. At applied stresses σ≥318 MPa, cracking on-loading prior to forward creep already occurs in the carburized layer, leading to a sharp increase in minimum creep rates of carburized specimens. These results demonstrate the impact a relatively thin layer of modified material can have on the materials creep performance; emphasises the requirement to consider the effects of the service environment in structural assessment procedures, particularly for high temperature applications including Generation IV and advanced modular reactor designs.