Abstract Charge states and lattice sites of Fe ions in virgin and Mn-doped Al _x Ga_1−x N samples were investigated using ⁵⁷Fe emission Mössbauer spectroscopy following radioactive ⁵⁷Mn⁺ ion implantation at ISOLDE, CERN. In the undoped Al _x Ga_1−x N, Fe²⁺ on Al/Ga sites associated with nitrogen vacancies and Fe³⁺ on substitutional Al/Ga sites are identified. With Mn doping, the contribution of Fe³⁺ is considerably reduced and replaced instead by a corresponding emergence of a single-line-like component consistent with Fe⁴⁺ on Al/Ga sites. Density functional theory calculations confirm the Fe⁴⁺ charge state as stabilised by the presence of substitutional Mn²⁺ in its vicinity. The completely filled spin up orbitals in Mn²⁺ (3d⁵) are expected to enhance magnetic exchange interactions. The population of the Fe⁴⁺ state is less pronounced at high Al concentration in Al _x Ga_1−x N:Mn, a behaviour attributable to hybridisation effects of 3d states to the semiconductor bands which weakens with increasing (decreasing) Al (Ga) content. Our results demonstrate that co-doping promotes the co-existence of unusual charge states of Fe⁴⁺ and Mn²⁺, whereas their trivalent charge states prevail with either transition metal incorporated independently in III-nitrides. Co-doping thus opens up a new avenue for tailoring novel magnetic properties in doped semiconductors.