The Monian Supergroup of western Anglesey comprises three major lithostratigraphic units: the South Stack (oldest), New Harbour and Gwna Groups. In northern and western Anglesey, deformation and metamorphism each increase in intensity downward through the Monian Supergroup, especially towards the base of the New Harbour Group. The variation in style and apparent intensity of deformation recorded by the South Stack and New Harbour Groups were governed by gross lithological differences. A correlation is proposed between the deformation histories of the New Harbour and South Stack Groups. Early deformation of the supergroup produced a pervasive bedding-parallel S1 fabric (chlorite) in both the New Harbour and, to a lesser extent, South Stack Groups. Development of the major, SE-verging F2 Penrhyn Mawr and Rhoscolyn anticlines, and associated minor folds also produced an axial planar S2 pressure solution and crenulation cleavage (phengite). The Rhoscolyn anticline was subsequently modified by the localized development of recumbent, mesoscale F3 folds with an axial planar pressure-solution cleavage (phengite to phengitic muscovite). The F3 event may have accompanied an episode of thrust faulting. Deformation of the South Stack and New Harbour Groups was accompanied by a progressive regional greenschist facies metamorphic event. The simplest interpretation of the deformation history of the Monian Supergroup involves a progressive SE-directed shear event which occurred prior to the deposition of an Ordovician overstep sequence on Anglesey.