A simple polymerization of trichlorophosphoranimine (Cl3P = N−SiMe3) mediated by functionalized triphenylphosphines is presented. In situ initiator formation and the subsequent polymerization progress are investigated by 31P NMR spectroscopy, demonstrating a living cationic polymerization mechanism. The polymer chain lengths and molecular weights of the resulting substituted poly(organo)phosphazenes are further studied by 1H NMR spectroscopy and size exclusion chromatography. This strategy facilitates the preparation of polyphosphazenes with controlled molecular weights and specific functional groups at the α‐chain end. Such well‐defined, mono‐end‐functionalized polymers have great potential use in bioconjugation, surface modification, and as building blocks for complex macromolecular constructs. Well‐defined, mono‐end‐functionalized polyphosphazenes are prepared via living cationic polymerization. Dichlorophosphoranes with (protected) functional groups are prepared in situ from commercially available triphenylphosphines and employed as initiators for the polymerization of trichlorophosphoranimine. NMR spectroscopy and SEC analysis show that the molecular weights can be controlled via the monomer:initiator ratio to give α‐chain‐functionalized poly(organo)phosphazenes for the preparation of macromolecular constructs.