Cu-based ternary chalcogenides have received significant interest as an alternative to conventional photovoltaic materials. CuInS2 and CuInSe2 are the most studied copper-based ternary chalcogenides for photovoltaics. Recently, copper tantalum sulfide (CuTaS3) has been proposed as a potential light absorber for photovoltaics. The synthesis conditions and growth mechanism of ternary systems control the final composition and crystal structure. However, such studies have not been explored for copper tantalum sulfides. Here, we studied the formation and growth mechanism of Cu3TaS4 nanocrystals (NCs) primarily using X-ray diffraction, transmission electron microscopy, and density functional theory (DFT) calculations. The reactions proceed via the formation of cubic Cu2-xS NCs due to soft Lewis acid - soft Lewis base interaction. The Cu2-xS have Cu vacancies, which can be controlled by the relative concentration of the Cu precursor. Ta incorporation occurs in the Cu2-xS NCs at Cu vacancy sites, followed by the diffusion of Ta by replacing Cu into the lattice. Low packing of atoms in Cu3TaS4 provides suitable diffusion channels for Ta and Cu atoms. The diffusion barriers of Ta atoms outweigh that of Cu atoms, implying a reaction rate controlled by Ta diffusion. Thus by varying the relative Cu precursor concentration, the concentration of Cu-vacancies in Cu2-xS can be tuned, which controls the growth rate of Cu3TaS4. Understanding of the growth mechanism obtained in this paper will significantly contribute to the rational synthesis of various Cu-based ternary chalcogenides that is not possible by direct synthesis and hence will have an impact on potential applications in photovoltaics and photocatalysis.