The magnetic properties of Mn₅Si₃C$_x$ and Mn₅Ge₃C$_x$ films prepared by magnetron cosputtering or C⁺-ion implantation are studied. The carbon-doped films exhibit ferromagnetic properties with Curie temperatures $T_C$ well above room temperature and metallic conductivity, making them possible candidates for future magnetic semiconductor-based devices. In Mn₅Si₃C$_x$, the carbon gives rise to a lattice expansion and a concomitant change of the magnetic order from antiferromagnetic Mn₅Si₃ to ferromagnetic Mn₅Si₃C_0.8 with $T_C$ = 350 K. Likewise, $T_C$ of ferromagnetic Mn₅Ge₃ is strongly enhanced in Mn₅Ge₃C_0.8. However, in this case the lattice is slightly compressed by carbon. This demonstrates that the effect of carbon on the magnetic behaviour in these compounds is not simply due to a change of the various interatomic distances by carbon but also due to a modification of the electronic band structure.