Dual-site transcranial magnetic stimulation has been widely employed to investigate the influence of cortical structures on the primary motor cortex. Here, we leveraged this technique to probe the causal influence of two key areas of the medial frontal cortex, namely the supplementary motor area and the medial orbitofrontal cortex, on primary motor cortex. We show that supplementary motor area stimulation facilitates primary motor cortex activity across short (6 and 8 ms) and long (12 ms) inter-stimulation intervals, putatively recruiting cortico-cortical and cortico-subcortico-cortical circuits, respectively. Crucially, magnetic resonance imaging revealed that this facilitatory effect depended on a key morphometric feature of supplementary motor area: individuals with larger supplementary motor area volumes exhibited more facilitation from supplementary motor area to primary motor cortex for both short and long inter-stimulation intervals. Notably, we also provide evidence that the facilitatory effect of supplementary motor area stimulation at short intervals is unlikely to arise from spinal interactions of volleys descending simultaneously from supplementary motor area and primary motor cortex. On the other hand, medial orbitofrontal cortex stimulation moderately suppressed primary motor cortex activity at both short and long intervals, irrespective of medial orbitofrontal cortex volume. These results suggest that dual-site transcranial magnetic stimulation is a fruitful approach to investigate the differential influence of supplementary motor area and medial orbitofrontal cortex on primary motor cortex activity, paving the way for the multimodal assessment of these fronto-motor circuits in health and disease.