In recent decades, indium nitride (InN) has been attracting a great deal of attention for its potential applicability in the field of light-emitting diodes (LEDs) and high-frequency electronics. However, the contribution from adsorption- and reaction- related processes at the atomic scale level to the InN growth has not yet been unveiled, limiting the process optimization that is essential to achieve highly crystalline and pure thin films. In this report, we investigate the reaction pathways that are involved in the crystal growth of InN thin film in atomic layer deposition (ALD) techniques from trimethylindium (TMI) and ammonia (NH3) precursors. To accomplish this task, we use a solid-state approach to perform the ab-initio calculations within the Perdew–Burke–Ernzerhof functional (PBE) level of theory. The results clarify the activation role from the N-rich layer to decrease the barrier for the first TMI precursor dissociation from Δ‡H= +227 kJ/mol, in gas phase, to solely +16 kJ/mol, in the surface environment. In either case, the subsequent CH3 release is found to be thermo- and kinetically favored with methylindium (MI) formed at the hcp site and ethane (C2H6) as the byproduct. In the following step, the TMI physisorption at a nearby occupied hcp site promotes the sequential hydrogen removal from the N-rich layer at the minimum energy cost of Δ‡H < +105 kJ/mol with methane (CH4) release. An alternative mechanism involving the production of CH4 is also feasible upon dissociation in gas phase. Furthermore, the high concentration of CH3 radicals, from precursor dissociation, might be the origin of the carbon impurities in this material under the experimental conditions of interest. Finally, the passivation methodology is not found to affect the evaluation of the surface-related processes, whereas the inclusion of spin-polarization is demonstrated to be essential to the proper understanding of the reaction mechanism.