Born in London, Rosenior started his career with Bristol City as a promising midfielder. Arguably his best moment came when he scored the second goal in a 2–0 victory for Bristol City against Carlisle United in the 2003 Football League Trophy Final at the Millennium Stadium. In the first round of Bristol City's cup run they defeated Queens Park Rangers on penalties, and it was Rosenior who scored the winning penalty in the shootout. In November 2003 he signed for Fulham.