Writing in 1868, the Philadelphia publisher-cum-historian Henry Charles Lea informed a friend, “I am trying to collect the materials for a history of the Inquisition.” The collecting of these materials—books, manuscripts, and copies of thousands of pages of documents housed in musty European archives and libraries—would occupy Lea (1825–1909) for the remainder of his life. It also led to publication of A History of the Inquisition of the Middle Ages (1884–87) and his acknowledged masterpiece, A History of the Inquisition of Spain (1906–7). Regarded as classics, these path-breaking books inaugurated better understanding of the history of an institution whose aims and methods troubled Lea and remain subjects of heated debate. The first biography of Lea since 1931, The Inquisition’s Inquisitor offers the most comprehensive review to date of his writing on the history of the Catholic Church. Though Lea is generally regarded as a leading practitioner of “scientific” history, Richard L. Kagan examines the extent to which Lea’s religious convictions compromised the ostensibly objective character of his work. Lea’s extensive surviving correspondence also enables Kagan to examine other aspects of Lea’s long and productive career as one of Philadelphia’s most prominent citizens. Lea appears here a young literary critic; a businessman who skillfully transformed his family’s publishing firm into the country’s leading producer of medical books; a dogged political reformer; and a philanthropist whose largesse benefitted many of Philadelphia’s cultural institutions. Newly discovered sources also allow for insights into Lea’s private life, notably his controversial infatuation with his first cousin and future wife, Anna C. Jaudon, and the periodic breakdowns that required abandonment of his beloved “intellectual pursuits.” The Inquisition’s Inquisitor concludes with a survey of Lea’s legacy with respect to current understanding of the Inquisition and to Philadelphia, where reminders of his accomplishments include an eponymous library at the University of Pennsylvania and public elementary school in nearby West Philadelphia.