Compressive creep experiments were utilized to investigate the influence of small additions of 0.25 at.% Mn and 0.10 at.% Mo on the creep resistance of a cast Al-0.08Zr-0.02Sc-0.01Er-0.10Si at.% alloy. The Mn- and Mo-modified alloy displays significantly enhanced creep resistance at 300 and 400 °C, due to solid-solution strengthening and the formation of two types of precipitates: Al₃(Zr,Sc,Er)(L1₂)-nanoprecipitates and α-Al(Mn,Mo)Si submicron platelets or cuboidal-shaped precipitates. The creep threshold stresses at 300 and 400 °C are 37 and 24 MPa, respectively, versus 19 and 15 MPa for the unmodified alloy. At 300 °C, the creep exponent n is found to change from 4.4 in the base alloy, to 3 in the modified alloy, consistent with a change from climb- to glide-controlled dislocation creep. The Mn- and Mo-modified alloy exhibits an as-cast grain-structure, which is finer (~0.35 mm versus 0.6 mm) and more equiaxed grains than the unmodified alloy, which is anticipated to enhance deformation by diffusional-creep. Nevertheless, diffusional-creep resistance at 400 °C remains high for the modified alloy, due to precipitation of submicron α-Al(Mn,Mo)Si-precipitates at grain boundaries (GBs). At 400 °C, the diffusional creep threshold-stress is ~14 MPa, three times that of the unmodified alloy, which also display fewer and coarser Al₃(Zr,Sc,Er)(D0₂₃) precipitates at GBs. Creep resistance in the modified alloy does not deteriorate after 16 days of stress testing at 400 °C, highlighting the excellent coarsening resistance of the L1₂- and α-precipitates. This new castable, heat-treatable aluminum alloy therefore represents an important technological advance for utilization at higher temperatures under stress.