In his final days in office in 1909, Theodore Roosevelt invited dozens of friends to the White House for lunch. They had never met as a group, but they had one thing in common: Each played tennis with the president and advised on policy matters. Roosevelt half-joked that the public would never know how much these tennis partners did to make his administration a success. Journalists dismissively called them the "Tennis Cabinet," making light of their contribution, but Roosevelt knew otherwise. This inner circle led the administration's campaigns against corporate greed, investigated public health violations, and formulated consumer protections. They founded environmental conservation policies, prosecuted civil rights violations, and implemented bureaucratic efficiencies that saved the government billions. Roosevelt's tennis mates shaped the nation's diplomacy, ending wars and promoting American interests abroad. Never had a more eclectic group advised a U.S. president. The Tennis Cabinet included legendary frontier lawman Seth Bullock and the starched-shirt corporate lawyer Henry Stimson, who served in five presidential administrations. Texas wolf wrangler Jack Abernathy played with stuffy bureaucrats like Labor Commissioner Charles Patrick Neill and social activist James Bronson Reynolds. The French ambassador Jean Jules Jusserand spun yarns with football hero George Washington Woodruff and Roosevelt's college friend and banker Robert Bacon. James Garfield, namesake son of a martyred president, sipped mint juleps with Supreme Court Justice William Henry Moody. And J. P. Morgan's silver-spooned son-in-law Herbert Satterlee kept company with rugged soldier Luther "Yellowstone" Kelly. For all their differences, these men shared a desire to help the president transform the nation from a parochial nineteenth-century republic into an imperial and industrial global power. They have escaped the attention of reporters and historians only because of Roosevelt's towering celebrity. Turning away from Roosevelt as the singular force behind his administration, it is possible to see how the contributions of his Tennis Cabinet quietly sowed the seeds of the American Century.