The influence of confinement in acidic nanotemplates on the polymerization reaction of pyrrole was previously reported. Similarly, an in situ oxidant-free polymerization of pyrrole on and in between the Ti3C2 MXene layers has been demonstrated experimentally. The newly formed PPy/MXene (PPy/polypyrrole) interface showed high electrical conductivity and supercapacitor features with excellent cycling stability. However, the polymerization mechanism remains unclear. In this study, the pyrrole polymerization mechanisms on Ti3C2 MXene surfaces and in between MXene interlayers with different terminations were investigated based on first-principles calculations. Key factors for such oxidant-free polymerization were identified, including the configuration of surface hydrogen-bonded pyrrole, surface acidity, confinement effect, and charge transfer between MXene surfaces and pyrrole monomers. By controlling these factors, one can design covalently bonded conducting organic molecules/polymers on or in between MXene interlayers for electronic applications. These findings not only uncover the mechanism for a proton-assisted pyrrole polymerization on and in between MXene interlayers but also provide a general mechanistic understanding and guideline for other possible in situ polymerizations on and in between two-dimensional MXene interlayers.