This study reported the development of a novel Al-Si-Mg-based composite reinforced by micron-sized Cu-Mn binary solid solution phases and submicron-sized α-Al(Mn,Fe)Si dispersoids. The Cu-Mn binary solid solution phases were added to the melt in the form of an Al-3%CuMn master alloy, whereas α-Al(Mn,Fe)Si dispersoids were obtained via heat treatment. The microstructure analysis confirmed the presence of micron-sized Cu-Mn binary, eutectic Mg₂Si, and Al₁₅(FeMn)₃Si₂ intermetallic phases, submicron-sized α-Al(Mn,Fe)Si dispersoids, and nano-sized precipitates in the Al-based composite. At room temperature, tensile results represented a yield strength of 287 MPa and a tensile strength of 306 MPa, with an elongation of 17%. Moreover, the Al-based composite maintained a yield strength of 277 MPa up to 250 °C, with a slight increase in elongation. The composite also exhibited excellent high-temperature high-cycle fatigue properties and showed a high-cycle fatigue limit of 140 MPa at 130 °C, which is ~2.3 times higher than that of the commercial A319 alloy. A fractography study revealed that the secondary particles hindered the movement of dislocations, thus delaying crack initiation under cyclic loading at high temperatures. Additionally, Cu-Mn binary solid solutions and Al₁₅(FeMn)₃Si₂ phases were found to be effective in reducing the crack propagation rate by hindering the movement of the propagated crack.