<p>Quantum tunneling is known to be mass dependent. Here we report an unexpected mass effect on tunneling reactions in isocyanic acid radical anions (HNCO^•−), which exhibit negative electron affinities. The <i>cis</i>- and <i>trans</i>-isomers of HNCO^•− were generated in a solid neon matrix and their interconversion and electron-detachment tunneling kinetics were investigated using infrared absorption spectroscopy. The results reveal that electron tunneling, which involves some degree of nuclear motion of HNCO^•− to neutral HNCO, occurs at a slower rate than the <i>cis</i>-to-<i>trans</i> isomerization of HNCO^•−, with the latter following a distinct bond angle inversion pathway driven by the heavy carbon atom. This contrasts to the typical dominance of light hydrogen atom tunneling in <i>cis</i>-<i>trans</i> isomerization systems. These findings are rationalized by instanton theory calculations. Our model, which explicitly includes the neon matrix via a QM/MM approach, reveals that the carbon-driven pathway is favored by a lower barrier and shorter tunneling distance.</p>