From 644, Lagny-sur-Marne was the site of
Lagny Abbey, a
monastery founded that year, and after its destruction by the Normans refounded about 990. The monastery was seized by the state at the
French Revolution and its buildings are used since 1842 as the offices of the municipality. During the
Middle Ages Lagny-sur-Marne was one of the most popular places for tourneys in Northern France. In November 1179 a notable
tournament was held by
Louis VII of France in honour of the coronation of his son. In 1170, the young knight
Baldwin of Bethune and his lifelong friend,
William Marshal were at the court of
Henry the Young King. In 1180 at the great international
tournament of Lagny, Baldwin was a
knight banneret, leading the Flemish team while William headed the English team. The town was one of the hosts of the
Champagne fairs in the 12th and 13th centuries. In 1846, Lagny-sur-Marne annexed the commune of Saint-Denis-du-Port. ==Demographics==