Abstract High-order harmonic generation (HHG) in solids offers a promising route to compact ultrafast sources of quantum light, but its efficiency and coherence are limited by ultrafast dephasing of laser-driven electron–hole polarization. Although widely described by a phenomenological coherence time, the microscopic origin of this dephasing has remained unresolved. Here, we combine wavelength-dependent HHG measurements in monolayer and bulk semiconductors with ab initio many-body calculations of electron–phonon scattering and strong-field simulations. We show that the measured wavelength scaling of the HHG yield is quantitatively reproduced only when semiconductor Bloch equations incorporate momentum-dependent dephasing derived from first principles scattering rates. The calculations reveal that strong-field-driven carriers rapidly enter regions of the Brillouin zone with enhanced electron–phonon scattering, giving rise to effective few-femtosecond dephasing. Our results establish a microscopic foundation for dephasing in solid-state HHG and provide a framework for engineering coherence in strongly driven solids for quantum photonic applications.