The introduction of nanoscale twin boundaries (TBs) is an effective strategy to achieve exceptional combination of superior strength, ductility and resistance to fracture, fatigue and wear. It has been demonstrated that the properties of nanotwinned metals are dominated by a number of deformation mechanisms unique to the interactions between dislocations and TBs. Here, we reveal yet a new type of dislocation mechanism called correlated necklace dislocations (CNDs). This mechanism controls the strengthening of nanotwinned metals as the twin thickness is reduced to around 1 nm. The presence of a cracklike defect as the dominant dislocation source could allow the same mechanism to operate at larger twin spacings. More importantly, we demonstrate that CNDs could be responsible for an unusual, history-independent and stable fatigue behavior of nanotwinned Cu containning highly oriented nanoscale twins. Our findings call for further theoretical and experimental investigations of the unique deformation mechanisms in nanotwinned metals.