As a youngster, Sean played for Top of the Hill Celtic in the
Derry and District League. As he developed, he broke into the Derry City team in the position of
striker and scored a
hat trick on his debut against Galway United. Hargan later converted to left back, where he offered unwavering commitment to the side, endearing him to the City supporters. Hargan has scored more than 50 competitive goals for the Candystripes, including the decisive header away to IFK Gothenburg in the 2006 UEFA Cup qualifying round; which remains the clubs most iconic moment in European competition. Also during the
2006 league season, Derry City escaped punishment for fielding a banned Hargan due to the league failing informing Derry City in the correct timeframe. After Derry's City's European experience which included memorable ties with Gretna and PSG, Shelbourne were reported to have shown interest in the left back, but Hargan pledged his loyalty and stayed with the Candystripes until the end of the 2008 season. With the signing of
Dave Rogers at the beginning of the 2007 season, Hargan's first-team place came under stiff competition. Nevertheless, due to his senior status in the team, he was handed the role of vice-captain to
Peter Hutton by
Pat Fenlon for the season and Hargan seen off the completion of Rogers until he sustained an injury that would keep him out for the rest of the season. Hargan was described as a loyal servant for the club, and by the end of the 2008 season, he had made a total of 415 appearances for Derry City - making him the club's third highest appearer ever. He signed for a Crusaders at the end of the 2008 season, after departing the Brandywell club after 13 years at the club. Hargan won eight major honours during his 13-year career at Derry City, including the 1996–97 League of Ireland Premier Division title, the FAI Cup in 2002 and 2006, and five League of Ireland Cups in 1995, 2005, 2006, 2007, and 2008. ==Crusaders==