Abstract Photocatalysis has emerged as a vital tool for purification of waste water and also to obtain alternate fuels (H2) through water splitting. TiO2, ZnO and other non-carbide systems (ZnS, metallic nanoparticles (NPs), etc.) have been studied thoroughly to enhance their photocatalytic performances through doping and the formation of heterojunctions. But, niobium carbide (NbC) is gaining interest of researchers due to its ability to catalyze electrochemical reactions (hydrogen evolution reaction and oxygen reduction reaction) along with excellent thermal and chemical stability. In the present work, NbC has been synthesized using conventional oxide precursor (Nb2O5), Mg metal powder and parthenium hysterophorous (PH; carrot grass, an agricultural hazardous weed) as carbon source at relatively low temperature (800 °C) as compared to conventional synthesis temperature (1200 °C). For the phase confirmation and morphological studies, X-ray diffraction and transmission electron microscopy have been done. It has been observed that the presence of NbCxOy/NbOz inside NbC nanoparticles induced the optical active sites resulting photocatalytic degradation of methylene blue (MB) dye under visible irradiation. With the help of various techniques (UV–visible, PL, XPS, RAMAN spectroscopies and scavenger tests), degradation mechanism has been established for the observed photocatalytic performance of synthesized NbCxOy/NbOz/NbCx/g-C heterostructure under optimized conditions. Further, for the confirmation of photodegradation of MB dye, mass spectrometry and TOC have also been carried out.