Display omitted •Engineered carbon-vacant CN (FCN) using formalin-assisted thermal polymerisation.•FCN achieved complete removal of Tetracycline via sonophotocatalysis within 60 min.•Boosted charge separation and extended visible light response over defected FCN.•Adsorption of oxygen molecules on FCN site resulted in abundant reactive species. Metal-free polymeric graphitic carbon nitride (CN) materials are robust and stable visible-light-driven photocatalysts that have recently piqued interest in photocatalytic applications. Its photocatalytic performance is restricted remarkably due to moderate oxidation ability and fast charge carrier recombination rate. To address these issues, we engineered carbon-vacant CN (FCN) using a facile formalin-assisted thermal polymerization of molten CN precursor in which the carbon vacancies (Cv) were regulated by altering formalin dosage. Consequently, FCN catalysts revealed Cv concentration-dependent sonophotocatalytic degradation of Tetracycline (TC) antibiotics over diverse water matrices. The optimal FCN exhibited complete TC degradation efficiency within 60 min with a synergy index of 1.4, which is approximately 2.6 times higher than that of pristine CN. The enhanced sonophotocatalytic performance was mainly due to the synergistic effect of ultrasound and light irradiation. The Cv formation also resulted in enhanced charge carrier transportation and facilitated oxygen adsorption at the Cv site of FCN - supported by both experimental study and theoretical calculation. Subsequently, FCN generated abundant reactive active oxygen species including, •O2–, as well as indirectly •OH which played a significant role in the degradation pathway and mineralisation of the TC molecules. This study provides insight into understanding the correlation between controllable defects and sonophotocatalytic degradation properties of the self-doped and deficient FCN.