Sebastian Sejean discovered American football through a supervisor from his college that invited him to attend training sessions for Molossian Asnieres-sur-Seine. In 2001, he went to Amiens where he joined the division, with hopes of joining the French Federation of American football, and eventually joined the Spartans club Amiens. In 2002, he was called to France junior team with which he participated in the European Junior Championships, where he earned the title of best player. In 2003, he was named in the Europe team to participate in the Global Championships. During this tournament he was noticed by U.S.A universities, including Louisiana Tech. That same year he participated with the senior team in France at the World Cup in Germany when France finished 4th. Then in 2004 he won the Diamond Helmet X with the Spartans. Thanks to his good performance he was offered to join Laval University in Quebec. In late 2004 he went to study in Canada then. At Laval, he joined the team of Red and Gold, establishing himself in the number of Red and Gold wins, he contributes to the Laval Dunsmore Cup in 2005 and 2007 and Vanier Cup in 2006. In September 2007, he participated in his second World Cup which took place in Japan. Sejean was asked to play in NFL Europa, he refuses the proposal made to him while hoping to get a contract in the Canadian League CFL. He was then approached by the Toronto Argonauts, but at the same time he was contacted to participate in the development program organized by foreign players in the NFL. Sejean decided to try his luck in the development program, where only 16 players Were offered contracts. During the tests that took place in London and Seville, he was confronted with sixty players, some of whom had played professionally in NFL Europa. Following the tests he was offered one of 16 precious contracts and signed with St. Louis Rams, becoming the first student from Laval to sign with a club in the NFL. In 2008, he contributed only to pre-season matches and training at the club. On August 9, Sejean came into play for the first time during the 4th quarter, he managed 2 tackles. Sejean played against the Chargers in San Diego, and added 3 tackles. Finally he played the last game of the pre-season against the Chiefs in Kansas City. At the end of the pre-season he had played 3 games with 6 solo tackles. Since the end of his contract with the Rams he has looked for a new club. At the beginning of 2011 season, he finally find a new team in Europe. Since 2011 he is playing in French and German league, most of his time is to train and play with the France National team. He won the Gold medal at Wroclaw during the World's Games in 2017. The next year 2018, he won the Gold medal at Vantaa during the European Championship.
