Recent studies have shown that nanotwinned copper (NT Cu) exhibits a combination of high strength and moderate ductility. However, most engineering and structural applications would also require materials to have superior fracture toughness and prolonged subcritical fatigue crack growth life. The current study investigates the effect of twin density on the crack initiation toughness and stable fatigue crack propagation characteristics of NT Cu. Specifically, we examine the effects of tailored density of nanotwins, incorporated into a fixed grain size of ultrafine-grained (UFG) copper with an average grain size of 450 nm, on the onset and progression of subcritical fracture under quasi-static and cyclic loading at room temperature. We show here that processing-induced, initially coherent nanoscale twins in UFG copper lead to a noticeable improvement in damage tolerance under conditions of plane stress. This work strongly suggests that an increase in twin density, at a fixed grain size, is beneficial not only for desirable combinations of strength and ductility but also for enhancing damage tolerance characteristics such as fracture toughness, threshold stress intensity factor range for fatigue fracture and subcritical fatigue crack growth life. Possible mechanistic origins of these trends are discussed, along with issues and challenges in the study of damage tolerance in NT Cu.