Familial hemiplegic migraine type 1 (FHM-1), a rare hereditary form of migraine with aura and hemiparesis, serves as a good model for exploring migraine pathophysiology. The FHM-1 gene encodes the pore-forming CaV2.1 subunit of human P/Q-type voltage-gated Ca2+ channels (VGCCs). Some FHM-1 mutations result in a decrease of whole cell P/Q-type current density in transfected cells/neurons. Questions remain as to whether and how these mutations may increase the gain of the trigeminal nociceptive pathway underlying migraine headache. Here, we investigated the effects of T666M, the most frequently occurring FHM-1 mutation, on VGCC currents and neuronal excitability in trigeminal ganglion (TG) neurons. We expressed human wild-type and T666M CaV2.1 subunits in cultured TG neurons from CaV2.1 knockout mice and recorded whole cell VGCC currents in transfected neurons. Currents mediated by individual VGCC subtypes were dissected according to their pharmacological and biophysical properties. TG neurons were sorted into three subpopulations based on their soma size and their affinity to isolectin B4 (IB4). We found that the T666M mutation did not affect total or surface expression of CaV2.1 proteins but caused a profound reduction of P/Q-type current in all subtypes of TG neurons. Interestingly, a compensatory increase in CaV3.2-mediated low-voltage-activated T-type currents only occurred in small IB4-negative (IB4−) TG neurons expressing T666M subunits. Current-clamp recordings showed that the T666M mutation resulted in hyperexcitability of the small IB4− TG population. Taken together, our results suggest a possible scenario through which FHM-1 mutations might increase the gain of the trigeminal nociceptive pathway.