This study developed a green and novel magnetic biochar via the co-pyrolysis of firwood biomass pre-treated with 10% (w/w) of either solid-phase (admixing; G10BCA) or liquid-phase (impregnation; G10BCI) goethite mineral (α-FeOOH). Newly fabricated magnetic biochars were characterized by inductively coupled plasma-optical emission spectroscopy (ICP-OES), Brunauer-Emmett-Teller (BET) equipment, X-ray powder diffractometry (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), proximate and elemental analyzer, and vibrating sample magnetometry. The effects of magnetic precursor, iron loading, and aqua-treatments on recoverability, magnetic property, and stability (resistance to α-FeOOH reconstructive crystallization/dissolution reactions) were explored and compared to those of magnetic biochar derived from conventional ferric chloride precursor (F10BCI). Results confirmed a direct correlation between biochar yields and ash contents with iron loading, irrespective of the used types of magnetic precursors (α-FeOOH or FeCl3). Although FeCl3 can generate magnetic biochar (F10BCI) with higher total carbon content (83.6%) and surface area (299 m2/g), α-FeOOH proved to be more effective at yielding magnetic biochars with nanostructured surfaces, lower water extractable components (thus green; G10BCA = 0.21 mg/mL and G10BCI = 0.16 mg/mL), higher magnetic saturation (G10BCA = 10.0 emu/g and G10BCI = 20.8 emu/g), higher ferromagnetic susceptibility, and excellent recoverability. α-FeOOH was undetected on the surface of G10BCA, post-aqua-treatments (over 30 days), and this demonstrated its stability in the face of demagnetization via α-FeOOH reformation reactions. Consequently, this study demonstrated that the admixing solid-phase α-FeOOH (10%) with firwood biomass offered a green, facile, and efficient way to thermochemically produce magnetic biochar. The produced biochar exhibited a superb stability to α-FeOOH reconstructive crystallization/dissolution reactions in aquatic (aqua) media, green attributes, good magnetic properties, and great potential applications in many areas of the economy.