This dissertation discusses the mezzo-soprano singer and her repertoire in the Parisian Opéra and Opéra-Comique companies between 1870 and 1918. Mezzo-sopranos are often cast in operas as secondary characters such as mothers, villains and teenaged boys, but they also have leading roles which can match the dramatic complexity of those of their soprano colleagues. Mezzo-soprano roles exist in all major operatic repertoires, but feature strongly in the French repertoire composed during the Third Republic (1870–1940). By analysing primary sources such as newspaper articles, contractual documents, correspondence, scores and images, this dissertation reconstructs the mezzo-soprano’s history in a pivotal time and geographical location, when mezzo-soprano-led works such as Bizet’s Carmen (1875), Saint-Saëns’ Samson et Dalila (1877), and Massenet’s Werther (1892) were enshrined in the operatic repertoire. Focusing primarily on five mezzo-sopranos — Célestine Galli-Marié (1840–1905), Blanche Deschamps-Jehin (1857–1923), Meyriane Héglon (1868–1942), Marie Delna (1875–1932) and Lucy Arbell (1879–1947) — I discuss the Third-Republic mezzo-soprano in these state-funded opera companies. I begin by examining the mezzo-sopranos’ techniques and education, and the realities of their professional lives in the companies. Next, I discuss Carmen, Samson et Dalila and Werther in the context of contemporary issues in the Third Republic, and how the core mezzo-sopranos of this dissertation interpreted their richly-drawn leading roles. Building from this, I finally explore the strong personal ties that three mezzo-sopranos had to their roles — Galli-Marié to Carmen, Delna to Marion in Godard’s La Vivandière (1895) and Arbell to the title role in Massenet’s Cléopâtre (premiered 1914) — and their effect on a work’s performance history.