Despite the increasing interest in the applications of functional nanoparticles, a comprehensive understanding of the formation mechanism starting from the precursor reaction with subsequent nucleation and growth is still a challenge. We for the first time investigated the kinetics of gold nanoparticle formation systematically by means of a lab-based in situ small-angle X-ray scattering (SAXS)/wide-angle X-ray scattering (WAXS)/UV–vis absorption spectroscopy experiment using a stopped-flow apparatus. We thus could systematically investigate the influence of all major factors such as precursor concentration, temperature, the presence of stabilizing ligands and cosolvents on the temporal evolution of particle size, size distribution, and optical properties from the early prenucleation state to the late growth phase. We for first time formulated and numerically solved a closed nucleation and growth model including the precursor reaction. We observe that the results can be well described within the framework of classical nucleation and growth theory, including also results of previous studies by other research groups. From the analysis, we can quantitatively derive values for the rate constants of precursor reaction and growth together with their activation free enthalpies. We find the growth process to be surface-reaction limited with negligible influence of Ostwald ripening yielding narrow disperse gold nanoparticles.