The regulated loading of the Mcm2–7 DNA helicase (comprising six related subunits, Mcm2 to Mcm7) into pre-replicative complexes at multiple replication origins ensures precise once per cell cycle replication in eukaryotic cells. The origin recognition complex (ORC), Cdc6 and Cdt1 load Mcm2–7 into a double hexamer bound around duplex DNA in an ATP-dependent reaction, but the molecular mechanism of this origin ‘licensing’ is still poorly understood. Here we show that both Mcm2–7 hexamers in Saccharomyces cerevisiae are recruited to origins by an essential, conserved carboxy-terminal domain of Mcm3 that interacts with and stimulates the ATPase activity of ORC–Cdc6. ATP hydrolysis can promote Mcm2–7 loading, but can also promote Mcm2–7 release if components are missing or if ORC has been inactivated by cyclin-dependent kinase phosphorylation. Our work provides new insights into how origins are licensed and reveals a novel ATPase-dependent mechanism contributing to precise once per cell cycle replication.