Garnet megacrysts found in the Cretaceous Group II Monteleo kimberlite (Free State province, South Africa) exhibit an unusual flattened and elongated morphology. Internally, the garnets are characterized by abundant kelyphite veins and microcracks, arranged as preferentially orientated microstructural sets. Numerous rod-like sulphide inclusions are present within the garnets, also characterized by a preferential orientation, with their long axes generally normal to the short axis of the host megacryst. The observed garnet megacryst morphologies and internal micro-structures are consistent with a shear-related deformation event, although the deformation mechanism remains unresolved. The deformation of the garnet megacrysts is inferred to have occurred prior to the entrainment of the garnets in their host kimberlite and is attributed to localized shearing in the lithospheric mantle, possibly associated with overlying crustal shearing that developed parallel to the Agulhas-Falkland Fracture Zone during Gondwana breakup. Variations in major and trace element concentrations indicate that the garnet megacrysts formed as a result of a simple fractional crystallization process, and pressure-temperature modelling indicates that they crystallized from a >1330°C melt at a depth of ~180 km, within a zone of metasomatism within the lithospheric mantle. A similarity in chemical composition between the megacrysts and garnets from associated lherzolitic lithologies suggests that percolation of the megacryst parental melt through the lithospheric mantle may have contributed to the metasomatism.