This study presents the development of a novel, eco-friendly magnetic nanocatalyst functionalized with graphene oxide. The magnetic nanocatalyst functionalized with graphene oxide was fabricated in five main steps under mild conditions. We evaluated the synthesized material's use as a heterogeneous base catalyst for facilitating one-pot, three-component reactions aimed at producing various phthalazine and dihydroquinazolin derivatives. These transformations were carried out under environmentally benign and mild conditions. The reactions proceeded efficiently in water, which served as an environmentally friendly solvent, and resulted in high product yields. Notable advantages of this method include its excellent yields, the use of a green solvent, a straightforward work-up process, short reaction times, and the catalyst’s ease of recovery and reuse. All the synthesized compounds have previously been reported in the literature and were identified based on their melting points and spectroscopic data, including those obtained through FT-IR, ¹H NMR, and ¹³C NMR analyses. The structure of the newly developed catalyst was thoroughly verified using a range of analytical techniques, including Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray spectroscopy (EDX), vibrating sample magnetometry (VSM), and thermogravimetric analysis (TGA). Furthermore, the catalyst demonstrated excellent reusability; it was recovered and reused across six reaction cycles with a minimal loss in performance.