A novel, active, and recyclable magnetic nano-size solid acid catalyst was developed for the high-yielding transformation of waste grease to biodiesel (fatty acid methyl esters, FAMEs) via simultaneous esterification of free fatty acids (FFAs) and transesterification of triglycerides with methanol in one pot. The core–shell structured magnetic nanoparticles (MNPs) HPW–PGMA–MNPs consist of iron oxide MNPs as the core, poly(glycidyl methacrylate) (PGMA) as the shell, and phosphotungstic acid (HPW) as the surface acid group. They were conveniently prepared in a 93% yield from PGMA–MNPs via phosphonation with Na₂HPO₄ and subsequent treatment with Na₂WO₄·2H₂O under acidic conditions, allowing for in situ formation of HPW on the particle surface with a high acidity of 1.13 mmol g⁻¹ and a particle size of 90 nm. The catalyst was fully characterized by EDX, FT-IR, FESEM, TEM, and VSM. It demonstrated a much better catalytic performance for the transesterification of triacetin and for the one-pot transformation of grease to biodiesel than commercially available solid acid catalysts such as Amberlyst 15, Purolite CT-275, and zeolite. One-pot transformation of grease (21.3 wt% FFAs) with methanol using HPW–PGMA–MNPs (4 wt%) gave a 98% FAME yield after 24 h, with 96% conversion for the esterification and >98% conversion for the transesterification. HPW–PGMA–MNPs were easily separated from the reaction mixture under a magnetic field and efficiently reused for further cycles of transformation, retaining 95% productivity in the 10^th reaction cycle. Thus, the developed magnetic nano-size solid acid catalyst is potentially useful for the green and economic production of biodiesel from waste grease.