A novel composite-composite joining technology based on metal pins oriented in through thickness direction of the composites is presented. A defined pin geometry, which is capable of establishing a through-thickness form-fit connection between composites and the metal reinforcement, is created on thin metal sheets in an automated pin production process. Based on numerical simulations of the fracture of unreinforced single lap shear (SLS) composite specimens, optimum locations for the pin reinforcement were found. Tests on reinforced SLS specimens proved that an enhanced damage tolerance can be achieved by the use of cold metal transfer welded pins (CMT pins) as through-the-thickness reinforcement of the joint area. This paper investigates the mechanisms responsible for the load transfer and failure of such through-the-thickness reinforced composite-composite joints during monotonic loading.