Self-forming barriers for copper (Cu) interconnect metallization are well developed. However, the self-forming-barrier process and associated electromigration behaviors of narrow lines of cobalt (Co), a new interconnect wiring material for sub-10-nm semiconductor device nodes, are yet to be explored. In this study, we present an all-wet electroless trench-filling process to fabricate narrow (100 nm) lines of Co and MnO-added Co [Co(MnO)], then thermally annealed to some extent (400 °C/30 min) prior to electromigration testing. Empirical data obtained from accelerated electromigration testing, including failure lifetimes, current-density scaling factors, activation energies and physical failure profiles, consistently show that electromigration reliability of the Co lines is markedly enhanced by 0.1% added MnO. Transmission electron microscopy and focused ion beam imaging, along with dynamic adhesion strength data, reveal that the enhancement is related to annealing-induced segregation and dispersion of MnO particles at the interface with SiO2, ensuring adhesion of Co on SiO2, preventing interfacial interdiffusion and stuffing electromigration pathways. The adhesion strength, microstructures and Joule-heating data of Co and Co(MnO) are provided for the discussion of the differences in their electromigration performance and failure mechanism. Comments on electroless-plated Co as a new interconnect material are also given.