Sengier is credited with giving the American government access to much of the uranium necessary for the Manhattan Project, much of which was already stored in a Staten Island warehouse due to his foresight to stockpile the ore to prevent it from falling into a possible enemy's hands.

[dubious – discuss] In May 1939, Sengier, then director of both the Société Générale and the UMHK, learned about the potential of uranium from English chemist Sir Henry Tizard, who warned him that he held "something which may mean a catastrophe to your country and mine if this material were to fall into the hands of a possible enemy."

Shortly thereafter, he was approached by a group of French scientists led by Frédéric Joliot-Curie, who asked whether Sengier would be willing to participate in their efforts to create a uranium fission bomb.

Since Tizard had informed him about uranium's potential a couple of years earlier, Sengier had a pretty good idea why Nichols had shown up to inquire about his ore deposits, something Nichols elaborated on during a 1965 Voices of the Manhattan Project interview by the journalist Stephane Groueff: He had been following some of the work done by the French scientists before the war, and he knew the importance of the uranium as a possibility.

"[8]In his 1962 book about the Manhattan Project, Now It Can Be Told, Groves wrote that "as a Belgian, Sengier appreciated fully the absolute necessity of an Allied victory.

The United States Army sent a squad from its Corps of Engineers to restore the mine, expand the aerodromes in Léopoldville and Elisabethville, and build a port in Matadi, on the Congo River.

To illustrate the uniqueness of Sengier's stockpile, after the war the MED and the AEC considered ore containing three-tenths of 1 percent as a good find.

Without Sengier’s foresight in stockpiling ore in the United States and aboveground in Africa, we simply would not have had the amounts of uranium needed to justify building the large separation plants and the plutonium reactors.

The uranium agreements in part explain Belgium's relative ease in rebuilding its economy after the war, as the country had no debt with the major financial powers.

[12] Sengier was described by author John Gunther in his book Inside Africa as "a tallish man, somewhat portly, with pale skin, white hands, a fringe of shinily white hair, and a short silver mustache clipped with sharp neatness", further stating that he "conveys that pleasant sense of benevolence which comes to an extremely successful man of affairs, after his major work is done.

[15] However, the official statement made note of Sengier's "sound judgment, initiative, resourcefulness and unfailing cooperation" which "contributed greatly to the success of the atomic bomb project".

So far as I know, no photograph of him has ever appeared in an American newspaper or magazine of wide circulation ..."[17] Sengierite, a radioactive mineral discovered in Congo in 1948, was named in his honor.