Photocatalytic degradation of organic pollutants using sillenite Bismuth ferrite-based materials is often limited by poor efficiency under low-power solar irradiation and the need for sensitizers to improve performance. To address this challenge, we synthesized a novel composite material based on sillenite-phase Bi25Fe0.5Ni0.5O40 (s-BNF) and pristine multi-walled carbon nanotubes (MWCNTs) via hydrothermal methods. Two different synthesis strategies were employed: the separable addition technique and the in-situ growth technique, producing composites denoted as s-BNF@(MWCNTs)x; [i.e. x = 0–5 wt.%]. The structural, morphological, magnetic, optical, and photocatalytic properties of the composites were investigated using X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Field Emission Scanning Electron Microscopy (FESEM), Transmission electron microscope (TEM), Ultraviolet–Visible Diffuse Reflectance Spectroscopy (UV–Vis DRS), and a Physical Property Measurement System (PPMS). XRD confirmed the formation of a single-phase cubic sillenite structure with no secondary phases upon MWCNT incorporation. FTIR suggested interfacial interactions between ferrite and MWCNTs, while magnetic measurements revealed progressive softening of magnetic properties with increasing MWCNT content. UV–Vis DRS analysis showed a significant reduction in the bandgap, particularly for the composite with 5 wt% MWCNTs prepared by the separable method. Photocatalytic tests demonstrated a substantial improvement in methylene blue degradation efficiency—from 21.45 % (pure s-BNF) to ∼43 % and ∼67 % for the two 5 wt% MWCNT composites—under very low-power solar light within 150 min, without addition of any external agent. This work presents a cost-effective and environmentally friendly strategy for enhancing visible-light-driven photocatalysis. This study aims to investigate different methods of MWCNT incorporation and analyze their comparative effectiveness in dye degradation.