A novel approach used to synthesize antimetabolite-conjugated and intense blue fluorescence-emitting smart polymeric nanoparticles is reported for the efficient delivery of anticancer drugs and self-monitoring their effect in drug-resistant metastatic breast cancer cells. Metastatic breast cancer is the deadliest cancer in women as chemotherapy does not perform well in its treatment. To prepare the drug-loaded fluorescent nanoparticles, the FDA-approved non-fluorescent poly(D,L-lactide-co-glycolide) (PLGA) polymer was modified into a newly designed fluorescent PLGA polymer by the covalent conjugation of the biocompatible fluorophore 1-pyrenebutyric acid (PBA). The fluorescent PLGA–PBA polymer was then electrosprayed by applying a potential of 8.0 kV to synthesize mono-dispersed spherical fluorescent nanoparticles (size ∼40 nm). The surface of the PLGA–PBA nanoparticles was conjugated with the potent anticancer drug molecule methotrexate (MTX) through a linker molecule, ethylenediamine (EDA), to kill cancer cells. The fluorescence, FTIR, NMR, and mass spectroscopy results of PLGA–PBA and PLGA–PBA@MTX nanoparticles provided proof of the successful synthesis of PBA- and MTX-conjugated nanoparticles with stable fluorescence for monitoring the in vitro therapeutic effect. A significant internalization of the PLGA–PBA@MTX nanoparticles was observed inside MTX-resistant MDA-MB-231 and MCF7 cells predominantly via micropinocytosis. Since MTX is an analog of folic acid and encourages cell internalization through the FRα receptor, a higher population of PLGA–PBA@MTX nanoparticles was observed in MDA-MB-231 cells with higher cell cytotoxicity than in MCF-7 cells. The amide bond, which links the MTX molecules to the surface of the fluorescent PLGA–PBA nanoparticles, was found to be sensitive to acidic pH. The controlled release of MTX occurred at pH 6.0 for ∼6 days due to the acid-catalyzed amide bond hydrolysis. The lower IC50 value justified a high MTX loading (36%) with significant conjugation efficiency (91.4%), and a rapid drug release in acidic medium compared to that of the free MTX molecule and high apoptosis in the nanoparticle-treated cells were observed. This study presents the usefulness of an engineered nano-formulation in pH-sensitive drug release and tracking of the therapeutic response, which may provide potential benefit in breast cancer treatment.