A wellbore mill system having a first mill with a main body with a top end and a bottom end, milling structure on the main body, and a hook portion projecting from the bottom end of the main body for supporting at least one member below the first mill, the hook portion having a lip. One such system has a whipstock having a top lug, the top lug resting on the lip of the hook portion of the first mill, the top lug having a hole therethrough, the hook portion of the main body of the first mill having a hole therethrough, a shear stud with a portion extending through the hole in the top lug and a portion extending through the hole in the hook portion thereby releasably securing the first mill to the whipstock.