Through the use of the tropes of intersectionality and transnationalism, this first-ever study of Jules Puech (1879–1957), is a double biography as it makes an intergenerational journey through his life’s work on Flora Tristan (1803–1844), feminist and socialist. Materials from the mid-nineteenth century press found from digitised searches extends knowledge of the advance of Flora Tristan’s political reputation. Its transmission beyond her notoriety as a radical during her lifetime was conveyed by both political activists and scholars. A key feature of the success of Puech is that he considered knowledge of her legacy as a significant ingredient of the nascent labour history of France of which he was part. My work claims that his biography was a major contribution to scholarship. It began when, as a postgraduate student in Paris in the 1900s, he completed his first doctoral thesis on Proudhonian influence on the first internationalist labour movements in France. My book explains the circumstances of how he embarked on the first in-depth biography of Flora Tristan and published it sixteen years later in 1925. By then Puech was unmatched in his knowledge of networks of activists who sustained the memory of early socialists, among them Flora Tristan. An independent scholar with a full-time job he was equally committed elsewhere. He and his suffragist feminist wife Marie-Louise, née Milhau, (1876–1966), also from a Protestant family of the Tarn, worked tirelessly for the pacifist movement, La Paix par le Droit. How his Flora Tristan study was thwarted by the wars of 1914–1918 and 1939–1945 is equally significant. In 1939, he handed both the original Flora Tristan journal and the typed manuscript of his edited Flora Tristan journal Tour de France to the newly established International Institute of Social History in Paris on the understanding that it would publish his work but was powerless to prevent their war-time disappearance. Their eventual recovery in Amsterdam came after his death, too late for him to see the fruition of his cherished project but available for trade-unionist Michel Collinet to publish his annotated edition in 1973, 130 years after Flora Tristan had begun to record her political campaign for a workers’ universal union. The double biography reveals both the multifaceted nature of feminism, socialism and pacifism in activism and the shaping of labour history as an academic subject in France of the first half of the twentieth century.