Rare earth elements (REEs), comprising lanthanides, scandium, and yttrium, are increasingly vital for advanced technologies, yet their separation remains challenging. This study presents a selective and efficient approach for cerium (Ce) recovery from REE concentrates via ex situ electrochemical oxidation coupled with solvent extraction in a continuous capillary system. Ce³⁺ is electrochemically oxidized to Ce⁴⁺ and subsequently extracted from La³⁺ using di(2-ethylhexyl)phosphoric acid (D2EHPA) in n-octane. Key extraction parameters, including capillary dimensions, flow regime, pH, and extractant concentration, were optimized, yielding >80% Ce⁴⁺ extraction within 5 min, with no detectable La³⁺ transfer. The method was further validated using a multielement REE mixture (Ce³⁺, La³⁺, Nd³⁺, Eu³⁺, and Yb³⁺) and applied to leachates from secondary sources such as polishing powder waste (PPW) and spent nickel–metal hydride (NiMH) batteries via changing extractants or using multistage extraction. The results underscore the general applicability of this redox-mediated separation process for sustainable REE purification. To further underscore its industrial relevance, this method avoids the need for complex ligands or temperature-sensitive procedures, making it suitable for future integration into modular recycling facilities.