Co-axial electrospinning is one of the facile methods for the fabrication of core-shell metal oxides for environmental applications. Indeed, core-shell architectures featuring a magnetic core and a photocatalytic shell represent a novel approach to catalytic nanostructures in applications such as water treatment and pollutant removal via magnetic separation. This study focuses on the fabrication of novel FeO-FeNiO/NiO core-shell nanofibers with enhanced optical and magnetic properties using co-axial electrospinning. The aim is to optimize the fabrication parameters, particularly the amount of metal precursor in the starting solutions, to achieve well-defined core and shell structures (rather than single-phase spinels), and to investigate phase transitions, structural characteristics, as well as the optical and magnetic properties of the resulting nanofibers. Raman, XRD, and XPS results show several phases and high defect concentration in the NiO shell. The FeO-FeNiO/NiO core-shell nanofibers exhibit strong visible-light absorption and significant magnetization. These advanced properties highlight their potential in photocatalytic applications.