The effect on crystalline perfection, second harmonic generation (SHG) efficiency, optical transparency and mechanical properties due to Mn(II) doping in KDP single crystals grown by slow evaporation solution technique by adding different quantities of MnCl2 in the solution has been investigated. The actual incorporated quantity of Mn(II) in the crystals was evaluated by atomic absorption spectroscopy. Powder XRD study confirms the crystal system of KDP and found no additional phases at all doping levels (1 to 5 mol%). The influence of Mn(II) doping on the crystalline perfection has been assessed by high-resolution XRD and these studies revealed that the grown crystals could accommodate Mn(II) in the interstitial positions of the crystalline matrix of KDP only up to some critical concentration, above which the crystal developed structural grain boundaries. The relative SHG efficiency of the crystals was found to be increased sharply at low concentrations (1 mol%), and above this value it was decreased as the concentration increases though it is still higher than that of pure KDP. UV-Vis studies also revealed the same behavior with a significant enhancement at 1 mol% concentration and later gradually decreased. Good increment in the hardness values has been observed by increasing the doping concentrations.