Ilmenite, being the main ore of titanium, is used as the raw material for the synthesis of titanium dioxide (TiO2). During the manufacturing process, a significant amount of TiO2 is wasted at different steps in the form of sludges. In this study, clarifier underflow sludge from a TiO2 production company was modified using nitrogen (N) doping via the hydrothermal method. Six samples of N-doped catalyst derived from ilmenite sludge were synthesized by varying the concentration of nitrogen source and calcination temperature, and their efficiency in degrading ciprofloxacin (CIP) was evaluated. The catalysts performed satisfactorily under visible light irradiation, and the effect of different operational parameters (pH, catalyst dosage, contaminant concentration, and contact time) on CIP degradation was investigated. Maximum degradation efficiency of 77.1% was obtained for the optimized catalyst under pH 5, catalyst dosage 1 g/L, contaminant concentration 10 mg/L, and contact time 150 min. The kinetic study of ciprofloxacin degradation using optimized parameters showed pseudo-first-order reaction kinetics. The study provides an insight into the utilization of clarifier underflow sludge, which is being treated as a waste, as a potential photocatalyst that performs under visible light in the degradation of persistent environmental contaminants.