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Metal‐rich borides with the Ti3Co5B2‐type structure represent an ideal playground for tuning magnetic interactions through chemical substitutions. In this work, density functional theory (DFT) and experimental studies of Ru‐rich quaternary borides with the general composition A2MRu5B2 (A=Zr, Hf, M=Fe, Mn) are presented. Total energy calculations show that the phases Zr2FeRu5B2 and Hf2FeRu5B2 prefer ground states with strong antiferromagnetic (AFM) interactions between ferromagnetic (FM) M‐chains. Manganese substitution for iron lowers these antiferromagnetic interchain interactions dramatically and creates a strong competition between FM and AFM states with a slight preference for AFM in Zr2MnRu5B2 and for FM in Hf2MnRu5B2. Magnetic property measurements show a field dependence of the AFM transition (TN): TN is found at 0.1 T for all phases with predicted AFM states whereas for the predicted FM phase it is found at a much lower magnetic field (0.005 T). Furthermore, TN is lowest for a Hf‐based phase (20 K) and highest for a Zr‐based one (28 K), in accordance with DFT predictions of weaker AFM interactions in the Hf‐based phases. Interestingly, the AFM transitions vanish in all compounds at higher fields (>1 T) in favor of FM transitions, indicating metamagnetic behaviors for these Ru‐rich phases. [ABSTRACT FROM AUTHOR]