As a young man, Lannoy had a military career, fighting against the Prince-Bishopric of Liège (1430), England (1436), Lorraine (1440) and the Electorate of Cologne (1447).
In later life he wrote to his son that when called upon to speak in sessions of the council he could feel self-conscious about his lack of education.
He became an ally of the Dauphin, the future Louis XI of France, during his exile in the Burgundian domains (1456–1461), and helped him claim his throne in 1461.
His closeness to the new king of France led to his deprivation from office in the Low Countries in 1462–1463, and in 1468 to open conflict with Duke Charles the Bold, who for a time forced him into exile.
In 1478 he negotiated with cities of Tournai and Cambrai, episcopal lordships of the Holy Roman Empire then under French protection.