The usage of p-arsanilic acid (ASA) in livestock and poultry farms leads to its release into the environment. The phototransformation of ASA generates toxic inorganic arsenic species and organic by-products, which affects the water security of the peripheral river basin. In this work, we have investigated the pH-dependent kinetics and mechanisms of ASA photolysis under Ultraviolet-C (UV-C) light (254 nm) irradiation in the pH range 1–11 in the absence of oxygen. The degradation rate of ASA, the ratio of the generated inorganic arsenic species, and the amount of primary organic by-products were determined under the aforesaid conditions. Our results clearly showed the pH-dependent photochemistry of ASA. The photolysis rate was low under acidic conditions, and As(III) and p-aminophenol were the main products. With increasing pH, the photolysis rate increased significantly, and As(V) and aniline became the main products. A pH-based kinetic model has been established that allows prediction of the generation rates of As(III), As(V), p-aminophenol, and aniline according to the photolysis rate of ASA.