In this study, we introduce a method for the retrieval of two-dimensional size-density distributions of noble metal alloy nanoparticles. The technique of analytical ultracentrifugation (AUC) equipped with a multi-wavelength extinction detector is used to measured coupled optical and sedimentation properties of the particles. The optical response of the nanoparticles is calculated using Mie’s theory, where the complex refractive index of the small noble metal nanoparticles is corrected due to the effect of reduced mean free path of electrons. Our method is firstly validated through the analysis of synthetic data. Furthermore, excellent agreement is found between scanning transmission electron microscopy and AUC retrieved experimental data. This comprehensive high throughput characterization approach can contribute to improve synthesis and scale up production of particulate systems as it provides a simple and fast method to determine noble metal alloy nanoparticle size and composition distributions simultaneously.