We present a new strategy, built upon the use of mussel-inspired polydopamine (PDA), for constructing multifunctional nanochains of magnetic nanoparticles. One key finding is that self-polymerization of PDA around magnetically aligned nanoparticles affords robust rigid magnetic nanochains with versatile reactivity imparted by PDA. In particular, we have shown that loading of metal nanoparticles on the nanochains via localized reduction by PDA gave rise to magnetically recyclable, self-mixing nanocatalysts. Surface coupling of PDA with nucleophilic thiol and amine groups via Michael addition and/or Schiff base reactions, on the other hand, enabled easy bioconjugation of targeting ligands such as DNA aptamer for specific recognition of the nanochains to cancer cells, which led to magnetolysis of the cancer cells in a spinning magnetic field. The PDA-enabled strategy allows for flexible selection of magnetic building blocks and post-synthesis functionalization, which are of considerable interest for a wide spectrum of chemical and biomedical applications.