A pupil of the humanist legal scholar Jacques Cujas at Bourges, Jeannin became an advocate in the Parlement (high court) of Burgundy in 1569 and its president in 1579. He opposed in vain the extension into his province of the massacre of Protestants begun on St. Bartholomew's Day, 1572. As a Roman Catholic, he supported Charles of Lorraine against Henry of Navarre (later Henry IV of France) and even went to Spain as an envoy for the Catholic League (1591). He worked, however, for the reconciliation of Charles and Henry, and he transferred his loyalty to Henry after the victory of Fontaine-Française (1595). By this time Henry, rightful king of France since 1589, had also abjured Protestantism, making Jeannin's switch easier. A member of the Council of State, he took a leading role in the politics of the new regime; for example, in negotiating the Treaty of Lyon with Savoy (1601) and as ambassador to the United Provinces (1607–09). After the assassination of Henry IV (1610), he was controller-general of finances for Marie de Médicis and regent for her son, King Louis XIII, until his dismissal in 1616. He was recalled to the government after the disgrace of Marie de Médicis (1617).