The substitution of Mn for Ru in SrRuO3 suppresses the ferromagnetic (FM) order monotonically at low concentration (<0.39) and induces antiferromagnetic (AFM) order at high concentration (>0.39). In an attempt to understand how the Mn substitution initially modifies the magnetic interactions in SrRu1−xMnxO3, a polycrystalline SrRu0.9Mn0.1O3 sample was prepared and studied by Mn55 and Ru99,101 spin-echo nuclear magnetic resonance (NMR). Mn55 NMR reveals that the Mn dopants are in an intermediate Mn3+∕4+ valence state due to fast electron hopping. The magnetic interaction between the Mn ions is FM, mediated by the double exchange mechanism. The Ru4+ hyperfine field is reduced due to an AFM coupling with the Mn moments, which shifts the Ru NMR peaks to lower frequencies. The suppression of the FM ordering temperature in SrRu1−xMnxO3 is attributed to the interruption of the itinerancy of the Ru 4d electrons by Mn substitution.