L-type Ca²⁺ channels possess a Ca²⁺-dependent inactivation (CDI) mechanism, affording feedback in diverse neurobiological settings and serving as prototype for unconventional calmodulin (CaM) regulation emerging in many Ca²⁺ channels. Crucial to such regulation is the preassociation of Ca²⁺-free CaM (apoCaM) to channels, facilitating rapid triggering of CDI as Ca²⁺/CaM shifts to a channel IQ site (IQ). Progress has been hindered by controversy over the preassociation site, as identified by in vitro assays. Most critical has been the failure to resolve a functional signature of preassociation. Here, we deploy novel FRET assays in live cells to identify a 73 aa channel segment, containing IQ, as the critical preassociation pocket. IQ mutations disrupting preassociation revealed accelerated voltage-dependent inactivation (VDI) as the functional hallmark of channels lacking preassociated CaM. Hence, the α_1C IQ segment is multifunctional—serving as ligand for preassociation and as Ca²⁺/CaM effector site for CDI.