Criticality experiments with 235U (metal and hydride) and 239Pu (metal) were performed during the Manhattan Project. Results from these experiments provided necessary information for the success of the Manhattan Project [LA-1033 to LA-1036 (1947), LA-02532-MS V I (1961), and Critical Assembly: A Technical History of Los Alamos During the Oppenheimer Years, 1943–1945 (1993)]. These experiments have been previously described in compilations made after the Manhattan Project, but those works are either lacking in technical details or are not publicly available. This work aims to provide detailed information while showcasing the enduring impact of these experiments 75 years after they were performed. Furthermore, we use modern computational methods embodied in the MCNP6® code and ENDF data to analyze and interpret these historic measurements. The world’s first four criticality accidents are also discussed, as lessons learned from these helped shape the field of criticality experiments.