West Ham United youth system managed by Tony Carr from the age of nine, becoming a 15-year-old first year scholar in 2000. He signed a three-year professional contract with the club on 1 August 2001, close to his 17th birthday. In the 2001–02 season, he made regular appearances for West Ham reserves, but was unable to break into the first team. He eventually signed an initial four-week loan deal with local rivals Millwall on 17 October 2002, making his First Division debut in an away loss against Norwich City on 19 October. His loan was twice extended by a month, before he was recalled early to Upton Park after having spent seven weeks at The Den, his final appearance in a Millwall shirt coming in another away defeat to Crystal Palace on 7 November. Johnson's Premier League and West Ham debut came on 22 January 2003 in another away defeat to a London club, this time against Charlton Athletic, when he came on for midfielder Édouard Cissé. He made 15 League appearances and one FA Cup appearance in 2002–03, becoming a more significant member of the West Ham team as the year went on. He even signed a new four-year deal to remain in east London on 18 March 2003, appearing keen to help keep the club in the top flight. However, his last outing in a Hammers shirt was to come on the final day of the season, 11 May 2003, in a 2–2 draw away at Birmingham City which condemned the Hammers to relegation to the First Division. Chelsea Two months after West Ham United's relegation, Johnson joined London rivals Chelsea, signing on 15 July for a fee of £6 million. The first purchase under the ownership of Roman Abramovich, he made his debut on 13 August in an away win against Žilina in the 2003–04 UEFA Champions League, and his first Chelsea goal came in the corresponding home leg two weeks later. His league debut came on 17 August in an Anfield win against Liverpool, but he had to wait until 9 November for his first league goal, netting the opener of a 5–0 victory over Newcastle United. He made 19 league appearances in 2003–04 as Chelsea finished as runners-up. In Chelsea's FA Cup fifth-round match against Newcastle United in the following season, goalkeeper Carlo Cudicini was sent off late in the match, and with no substitutions