The microstructural evolution of Cu−19Ni−6Cr−7Mn alloy during aging treatment was investigated. After aging for 120 min at 500 °C, the alloy exhibited excellent mechanical properties, including a tensile strength of 978 MPa and an elastic modulus of 145.8 GPa. After aging for 240 min at 500 °C, the elastic modulus of the alloy reached 149.5 GPa, which was among the highest values reported for Cu alloys. It was worth mentioning that the tensile strength increased rapidly from 740 to 934 MPa after aging for 5 min at 500 °C, which was close to the maximum tensile strength (978 MPa). Analysis of the underlying strengthening mechanisms and phase transformation behavior revealed that the Cu−19Ni−6Cr−7Mn alloy underwent spinodal decomposition and DO₂₂ ordering during the first 5 min of aging at 500 °C, and L1₂ ordered phases and bcc-Cr precipitates appeared. Therefore, the enhanced mechanical properties of the Cu−19Ni−6Cr−7Mn alloy can be attributed to the stress field generated by spinodal decomposition and the presence of nanoscale ordered phase and Cr precipitates.