Upon the club's return to the top division, Harris solidified his reputation as an uncompromising – yet talented – defender with a series of strong (and sometimes notorious) performances. His first honours with Chelsea came with a League Cup win over Leicester City in 1965. In the same season, Chelsea were challenging for the league title for most of the year but ultimately finished third after winning just one of their final five matches. He became club captain the following year when Terry Venables left for Tottenham Hotspur and became the youngest ever captain to lead out a side in the 1967 FA Cup Final, although they lost 2–1 to Tottenham. Chelsea, led by Harris, reached another FA Cup final three years later, this time against Leeds United – a side then at their peak in English football. He won four caps for England U23 between 1966 and 1968.