|Abstract||Between 1865 and 1945, a number of prominent marriages united American heiresses and members of the British aristocracy. Through the lives of Lady Jennie Jerome Churchill, Duchess Consuelo Vanderbilt Balsan, and Lady Nancy Astor, this dissertation analyzes the lives and marriages of American-born, British-wed women within the context of diplomatic service and the Anglo-American rapprochement as they demonstrated a keen ability to de-masculinize the traditionally male world of diplomacy. These women surprised their families--both British and American--and often themselves--as they exhibited an extraordinary degree of agency in a period that clearly placed women outside the boundaries of politics and diplomacy. Their positions as the wives of leading members of the British aristocracy provided them with unprecedented access to the eyes and ears of individuals at the highest level in Great Britain, the very decision-makers who formulated and implemented foreign policy with their home country. During the period under consideration, Americans and Britons began to view one another less as adversaries and more as allies. Through their marriages, these women skillfully and successfully blurred the lines of public politics and private lives in a period that did not afford women the right to vote. Without formal educations in politics or foreign policy, without the title or staff provided to a diplomat or ambassador, these women created an unprecedented degree of agency within a world that would have undeniably recoiled at the idea of a female diplomat or politician. Collectively and individually, these women informal ambassadors who worked to improve relations at the turn of the twentieth century and served an important role in terms of influencing foreign relations as the United States and Britain moved toward the "special relationship."