A breakaway accessory mounting assembly for vehicles for supporting rearview mirrors and other accessories includes a base mounted on a vehicle panel surface such as a window, and a coupler engaging the base and having a support mounting the rearview mirror or other accessory. Retaining means such as a wire or stamped metal spring, or a resilient plate extend between the base and coupler and allow installation by slidably receiving the coupler on the base in a lateral direction followed by rotation of the coupler with respect to the base to an installed position about a predetermined first axis without further lateral sliding movement. The retaining means also resiliently hold the base and coupler together in the installed position, and allow release of the coupler from the base in three different separation modes when a predetermined force is imposed on the coupler or supported mirror or other accessory. Positioning means are provided to position the coupler in the installed position on the base.