The influence of carbon concentration on the electronic and magnetic properties of C-implanted ZnO thin films has been studied using synchrotron radiation based X-ray absorption spectroscopic techniques and vibrating sample magnetometer measurements. 20 keV carbon ions were implanted in ZnO films with different fluences (2 × 1016, 4 × 1016 and 6 × 1016 ions per cm2). The pristine ZnO film shows diamagnetic behaviour while the C-implanted films exhibit room temperature ferromagnetism. Our first-principles calculations based on density functional theory show an appreciable magnetic moment only when the implanted C atom sits either in the O-site (2 μB) or in the interstitial position (1.88 μB), whereas the C atom in the Zn substitutional position does not possess any magnetic moment. X-ray absorption near edge structure analysis at the O K-edge reveals that the charge transfer from O-2p to the C-defect site causes the ferromagnetism in the C-implanted ZnO film at low fluence. However at high fluence, the implanted C replaces the lattice and produces more Zn vacancies, as evidenced by extended X-ray absorption fine structure studies at the Zn K-edge, which favors the ferromagnetism. The persistence of the implanted carbon and ferromagnetism of the C-implanted ZnO film has also been studied by isothermal annealing at 500 °C and discussed in detail.