Influence of growth temperature on swift heavy ion (SHI) induced structural and optical functionality in CdS thin films is explored for photonic applications. Intense green emission is observed in nanocrystalline CdS thin films grown by pulsed laser deposition (PLD) at two different substrate temperatures (Ts): room temperature (RT) and 200 ºC. The role of Ts and its implications on the effect of dense electronic excitation provoked by swift heavy ion irradiation (SHII) on various optical and structural properties of CdS films is investigated under the influence of 70 MeV 58 Ni +6 ion beam. It reveals from the present studies that Ts may crucially affect the crystalline structure, vibrational and electronic states of the film and thereafter the functionality induced by ion beam. It is found that ion beam is capable to transform structural phase from mixed phase of cubic and hexagonal structure to either pure cubic or pure hexagonal phase of CdS depending upon the pre-existing preferred orientation in pristine film. The modification in crystallite size and band gap due to impact of ion beam is found to be strongly dependent on pre-existing structural phase, as determined by Ts. The studies presented here confirm that initial growth conditions play a key role even after post deposition SHII treatment in selecting precisely the functional behavior of the films.