Advanced thermo-mechanical processing of mild steels in the ferrite phase field has recently achieved breakthrough in grain refinement into the submicron regime. However, these steels often suffer from grain boundary failure and low rates of work hardening. A potential approach to overcome these challenges is to process modern high-strength low-alloy steels with multi-scale hierarchical microstructures. Thus, the applicability of advanced thermo-mechanical processing for achieving such microstructures in a high-strength low-alloy steel was studied. The microstructural evolution during warm deformation of a martensitic/bainitic starting microstructure using a Gleeble 3500 thermo-mechanical simulator at 600 °C followed by a direct aging step was investigated. The strain rate of 10 s−1led to strain localization and, therefore, the formation of a macroscopic shear band. High-resolution characterization techniques such as electron channeling contrast imaging, electron backscatter diffraction, and transmission electron microscopy were used to reveal the ultrafine grain sizes (~ 0.5 μm) in this shear band. The mechanism behind this refinement is continuous dynamic recrystallization, as the initial grains subdivided into smaller crystallites that are confined by a mix of subgrain and high-angle grain boundaries. Two populations of precipitates were formed. Larger precipitates (mean diameter ~ 150 nm) decorate grain boundaries, whereas smaller precipitates (~ 15 nm) nucleate on dislocations and subgrain boundaries.