We report on both experimental and modeling of carbon and nitrogen segregation to dislocations in C–Mn ferritic steels. A model based on McLean theory is established to describe the competitive segregation of carbon and nitrogen in the Cottrell atmospheres around dislocations, considering the effect of dislocation density. It is expected from the model that a high dislocation density strongly depletes interstitial atoms from the matrix, resulting in a low excess quantity of atoms on dislocations. Atom probe tomography analyses performed on two different C–Mn weld steels are in accordance with the model: carbon excess is tenfold higher in comparison to nitrogen excess in the deposited metal. However, after tensile test up to failure, this trend is no longer valid and nitrogen segregates preferentially than carbon. This suggests that plastic strain can modify the material properties by changing the nature and quantity of segregated atoms around dislocations.