55Mn NMR was used to investigate the effect of carbon doping on the local magnetic anisotropy in Mn5Ge3 epitaxial films (space-group P63 /mcm). It was found that carbon enters interstitially in the vicinity of the 6(g) crystallographic positions, occupying the 2(b) octahedral voids. The magnetic properties of the Mn atoms located in the corners of a host octahedron are strongly modified by the presence of carbon. Their magnetic moment is reduced by 0.7μB with respect to the pristine Mn5Ge3 film and the anisotropy of their orbital moment, measured as a difference between its value along the hexagonal c direction and on the c plane is reduced to 0.058μB , whereas, in the pristine Mn5Ge3 films, it oscillated every 60° between 0.151μB and 0. These changes are responsible for a significant decrease in magnetocrystalline anisotropy, which was reported to drop by an order of magnitude upon doping the Mn5Ge3 films with carbon.