The surge in industrial activities and a substantial increase in antibiotic usage, has led to the further deterioration of global water pollution problems. Focusing on this current scenario, we synthesize the g-C3N4-MWCNT/In2O3 (GMI) ternary composite to utilize for the photocatalytic degradation of levofloxacin (LF) antibiotic and crystal violet (CV) dye under visible light. The photocatalytic degradation of CV (15 mgL⁻1) and LF (40 mgL⁻1) was 99.7 % and 87.4 % respectively, in just 15 and 100 min of irradiation with GMI-0.2 nanocomposite. The removal rates of CV and LF with GMI-0.2 nanocomposite were 14.61 and 4.85 times more rapid than those observed with In2O3. Similarly, compared to g-C3N4, the removal rates for CV and LF were 7.34 and 3.98 times higher when using GMI-0.2 nanocomposite. The trapping experiments and ESR measurements indicated that O2•⁻ was the predominant active species responsible for the photocatalytic degradation processes. In order to explain the degradation processes, a feasible photocatalytic mechanism for GMI ternary composite was proposed. The mechanism was based on the calculations of the band gaps and edge potentials of the synthesised materials as well as the findings of the quenching experiments. The GMI-0.2 nanocomposite was also used to treat human lung cancer cells, and it was discovered that cell viability with 100 µg/mL of the photocatalyst was lowest in both dark (36 %) and in visible light (20 %).