Cratering and plastic deformation induced by individual MeV ions on the surface of polystyrene thin films of different molecular weights ${(M}_{w})$ (from 3250 to $2\ifmmode\times\else\texttimes\fi{}{10}^{7}u)$ are investigated using scanning force microscopy. 20 MeV, 85 MeV and 197 MeV gold ions are used to bombard the targets at grazing incidence $(79\ifmmode^\circ\else\textdegree\fi{}$ to the surface normal). Induced surface tracks consist of an elliptical crater followed by a hillock elongated in the direction of the ion incidence. For a given ion energy, the crater size is largest on the lowest ${M}_{w}$ film. Crater dimensions are systematically reduced on films of heavier macro-molecules, up to a molecular weight of about $1.6\ifmmode\times\else\texttimes\fi{}{10}^{5}u.$ For ${M}_{w}>1.6\ifmmode\times\else\texttimes\fi{}{10}^{5}u,$ the crater size remains approximately constant. The difference observed for the lateral dimensions of the craters are about 50% when comparing the lowest and the highest ${M}_{w}$ films at a fixed energy. The observed saturation of the crater size for high ${M}_{w}$ values coincides with the onset of entanglement effects in the polymer, which influences the viscosity and the compliance of the material. Moreover, the curve of the crater size versus ${M}_{w}$ follows the same trend as the reciprocal viscosity $({\ensuremath{\eta}}^{\ensuremath{-}1})$ versus ${M}_{w},$ indicating that the viscosity is governing the final lateral dimensions of the craters. The hillock dimensions present a weak dependence on ${M}_{w},$ above a threshold at $3250u.$ The different behavior observed for craters and hillocks is discussed based on the viscoelastic properties of the polymer at different ${M}_{w}$ and on the transient heating occurring close to the ion impact site.