To clarify functions of the Mre11/Rad50 (MR) complex in DNA double-strand break repair, we report Pyrococcus furiosus Mre11 crystal structures, revealing a protein phosphatase-like, dimanganese binding domain capped by a unique domain controlling active site access. These structures unify Mre11's multiple nuclease activities in a single endo/exonuclease mechanism and reveal eukaryotic macromolecular interaction sites by mapping human and yeast Mre11 mutations. Furthermore, the structure of the P. furiosus Rad50 ABC-ATPase with its adjacent coiled-coil defines a compact Mre11/Rad50-ATPase complex and suggests that Rad50-ATP-driven conformational switching directly controls the Mre11 exonuclease. Electron microscopy, small angle X-ray scattering, and ultracentrifugation data of human and P. furiosus MR reveal a dual functional complex consisting of a (Mre11)2/(Rad50)2 heterotetrameric DNA processing head and a double coiled-coil linker.