The introduction of magnetism in two-dimensional (2D) materials represents an intense field of research nowadays and the quest to reach above-room-temperature ordering temperatures is still underway. Intrinsic ferromagnetism was discovered in 2017 in CrI3 and Cr2Ge2Te6 in the monolayer form with low Curie temperatures. An alternative method to introduce magnetism into conventional 2D materials is substitutional doping with magnetic impurities similarly to three-dimensional diluted magnetic semiconductors. The case of Mn-doped transition metal dichalcogenide (MoS2, MoSe2, WS2, WSe2) monolayers is very interesting because combining out-of-plane ferromagnetism and valley contrast leads to ferrovalley materials. In this work, we focus on the incorporation of Mn in MoSe2 by molecular beam epitaxy on graphene which has been rarely addressed up to now. By using a multiscale characterization approach, we demonstrate that Mn atoms are incorporated into the MoSe2 monolayer up to 5 atomic percent. However, when incorporated into the film, Mn atoms tend to diffuse to the grain edges forming undefined MoxMnySez phase at grain boundaries after completion of the MoSe2 monolayer. This segregation leaves the crystalline and electronic structure of MoSe2 unmodified. Above 5%, the saturation of Mn content in MoSe2 leads to the formation of epitaxial MnSe clusters. © Académie des sciences, Paris and the authors, 2021. Some rights reserved.