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The critical role of silicon and germanium in the semiconductor industry, combined with the need for extremely thin channels for scaled electronic devices, has motivated research towards monolayer silicon (silicene) and monolayer germanium (germanene). The lack of horizontal mirror (σh) symmetry in these two-dimensional crystals results in a very strong coupling—in principle diverging—of electrons to long wavelength flexural branch (ZA) phonons. For semi-metallic Dirac materials lacking σh symmetry, like silicene and germanene, this effect is further exacerbated by strong back-scattering at the Dirac cone. In order to gauge the intrinsic transport limitations of silicene and germanene, we perform low- and high-field transport studies using first-principles Monte-Carlo simulations. We take into account the full band structure and solve the electron-phonon matrix elements to treat correctly the material anisotropy and wavefunction overlap-integral effects. We avoid the divergence of the ZA phonon scattering rate through the introduction of an optimistic (1 nm long wavelength) cutoff for the ZA phonons. Even with this cutoff for long-wavelength ZA phonons, essentially prohibiting intravalley scattering, we observe that intervalley ZA phonon scattering dominates the overall transport properties. We obtain relatively large electron mobilities of 701 cm2 V−1 s−1 for silicene and 2327 cm2 V−1 s−1 for germanene. Our results show that silicene and germanene may exhibit electronic transport properties that could surpass those of many other two-dimensional materials, if intravalley ZA phonon scattering could be suppressed. [ABSTRACT FROM AUTHOR]