L‐type voltage‐gated calcium (Cav1) channels have a key role in long‐term synaptic plasticity, sensory transduction, muscle contraction and hormone release. A point mutation in the gene encoding Cav1.2 (CACNA1C) causes Tymothy syndrome (TS), a multisystem disorder featuring cardiac arrhythmias, autism spectrum disorder (ASD) and adrenal gland dysfunction. In the more severe type‐2 form (TS2), the missense mutation G406R is on exon 8 coding for the IS6‐helix of the Cav1.2 channel. The mutation causes reduced inactivation and induces autism. How this occurs and how Cav1.2 gating‐changes alter cell excitability, neuronal firing and hormone release on a molecular basis is still largely unknown. Here, using the TS2‐neo mouse model of TS we show that the G406R mutation altered excitability and reduced secretory activity in adrenal chromaffin cells (CCs). Specifically, the TS2 mutation reduced the rate of voltage‐dependent inactivation and shifted leftward the activation and steady‐state inactivation of L‐type channels. This markedly increased the resting ‘window’ Ca²⁺ current that caused an increased percentage of CCs undergoing abnormal action potential (AP) burst firing, cell swelling, reduced mitochondrial metabolism and decreased catecholamine release. The increased ‘window’ Ca²⁺ current caused also decreased Na_V channel density and increased steady‐state inactivation, which contributed to the increased abnormal burst firing. Overnight incubation with the L‐type channel blocker nifedipine rescued the normal AP firing of CCs, the density of functioning Na_V channels and their steady‐state inactivation. We provide evidence that CC malfunction derives from the altered Cav1.2 channel gating and that dihydropyridines are potential therapeutics for ASD.