Fascinated by comics, Sannois-born Sylvain Vallée cooperated on fanzines like Les Cahiers Ludiques and took courses at the Institut Saint-Luc in Brussels. He returned to France in 1996, where he drew his first comic books containing short stories ('A ce soir, Chérie', 'Plein ma Brouette'), that were published by Oh, la Vache! in Rennes. He was also involved in the A.L.A.B.D., the organisation behind the Laval comics festival and several expositions. He additionally served as a press and advertising illustrator for Haut Anjou and Ouest-France, while also organizing comics courses for children.
While still at school and drawing for the press, he wrote and drew his first long comic, 'L'Écrin', that was published by Éditions du Cycliste in 1997. He continued his association with this publisher by making illustrations for posters inspired by famous films. By 1999, Vallée took over artwork of Kraehn's 'Gil Saint-André' series. Kraehn remained attached to the series as the writer.
Also for Glénat, he also teamed up with Fabien Nury, with whom he made 'Il était une fois en France', about the life of the Rumanian Jew Joseph Joanovici in France during World War II. for Glénat in 2006.