Lagny-sur-Marne is twinned with Sainte-Agathe-des-Monts, Quebec in Canada since 1969, and also with Alnwick, Northumberland in England.
Lagny-sur-Marne kembar dengan Sainte-Agathe-des-Monts, Quebec di Kanada sejak 1969, dan juga Alnwick, Northumberland di Inggris.