Multifunctional magnetic polymer nanocombinations are gaining importance in cancer nanotheranostics due to their safety and their potential in delivering targeted functions. Herein, we report a novel multifunctional core–shell magnetic polymer therapeutic nanocomposites (NCs) exhibiting pH dependent “Off–On” release of drug against breast cancer cells. The NCs are intact in blood circulation (“Off” state), i.e., at physiological pH, whereas activated (“On” state) at intracellular acidic pH environment of the targeted breast cancer cells. The NCs are prepared by coating the cannonball (iron nanocore) with hydrophobic nanopockets of pH-responsive poly(d,l-lactic-co-glycolic acid) (PLGA) polymer nanoshell that allows efficient loading of therapeutics. Further, the nanocore–polymer shell is stabilized by poly(vinylpyrrolidone) (PVP) and functionalized with a targeting HER2 ligand. The prepared Her–Fe3O4@PLGA–PVP nanocomposites facilitate packing of anticancer drug (Tamoxifen) without premature release in the bloodstream, recognizing the target cells through binding of Herceptin antibody to HER2, a cell surface receptor expressed by breast cancer cells to promote HER2 receptor mediated endocytosis and finally releasing the drug at the intracellular site of tumor cells (“On” state) to induce apoptosis. The therapeutic efficiency of hemo/cytocompatible NCs drug delivery system (DDS) in terms of targeted delivery and sustained release of therapeutic agent against breast cancer cells was substantiated by in vitro and in vivo studies. The multifunctional properties of Her–Tam–Fe3O4@PLGA–PVP NCs may open up new avenues in cancer therapy through overcoming the limitations of conventional cancer therapy.