We report the synthesis of Ag–Au alloy nanoparticles (NPs) by post-irradiation of mixed colloidal Ag and Au NPs by a nanosecond pulse laser in the presence of an external electric field (EEF). In our study, size, optical, and plasmonic properties of colloidal Ag–Au alloy NPs were investigated. A colloid of individual Ag and Au NPs were separately prepared by nanosecond pulsed laser ablation in distilled water. An appropriate volume of these colloidal NPs samples was mixed and post-irradiated with the same pulsed laser beam in the presence of a uniform electric field, resulting in the formation of Ag–Au alloy NPs. The synthesized colloidal alloy NPs were characterized using scanning electron microscopy and UV–vis absorption spectroscopy. The results indicate that the EEF can reduce significantly the Ag–Au alloy formation time. The results also show that the wavelength of the localized surface plasmon resonance of alloy NPs decreased by the applied electric field.