Third harmonic generating nonlinear optical (NLO) mixed crystals of dimethylamino pyridinium p-bromo-chlorophenolate belonging to the monoclinic crystal system were grown by slow evaporation method. From the recorded UV–Vis transmission spectrum, the optical band gap was determined to be 3.581 eV. The assessed values of nonlinear refractive index (n 2 ), nonlinear absorption coefficient (β) from the Z-scan studies shows the appropriateness of the grown mixed crystal for NLO applications. The real and imaginary parts of the third order nonlinear optical susceptibility (χ (3) ) were found to be 4.801 × 10 −6 esu and 0.102 × 10 −6 esu. The material exhibits a negative refractive index resulting in self-defocusing nature. The laser damage threshold value for DMAPBCP mixed crystal was acquired to be 2.98 GW/cm 2 . Positive photoconductivity of the crystal has been revealed. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.