ZnO nanoparticles were synthesized under mild hydrothermal conditions (T = 150 °C, P = autogenous, experimental duration = 18 h). Manganese was added as an additive to ZnO nanoparticles in different molar percentages. In situ surface-modification was successfully carried out for these manganese-added ZnO nanoparticles using n-butylamine as a surface modifier. The modified manganese-added ZnO nanoparticulates are hydrophilic in nature and are well dispersed in various solvents. The modified nanoparticles were characterized using powder XRD, FTIR, SEM, Zeta potential, and UV–Vis spectrophotometry. The characterization results indicated tailoring of the morphology and size of the nanoparticles, and changing the surface chemistry of the nanoparticles synthesized. The SEM results show that the surface modified manganese-added ZnO nanoparticles have a very thin layer of organic coverage around the inorganic nanoparticles, thus, giving rise to hybrid nanoparticles. The photodegradation of Brilliant Blue dye under sunlight showed the higher efficiency of the modified manganese-doped ZnO nanoparticles compared to the reagent-grade ZnO.