Harris is Millwall's all-time record goalscorer, with 138 goals in all competitions. He broke the previous record of 111 goals, held by Teddy Sheringham, on 13 January 2009, during a 3–2 away win at Crewe Alexandra. He has made the fourth most appearances for the club, with 432. He also played for Cambridge City, Cardiff City, Nottingham Forest, Gillingham and Southend United. Harris retired from professional football in June 2013 and took up a coaching role at Millwall. Having briefly acted as caretaker-manager after the dismissal of Steve Lomas in January 2014, Harris was given the same role following the dismissal of Ian Holloway in March 2015 and was confirmed as permanent manager of Millwall on 29 April 2015. He moved on to become manager of Cardiff City in November 2019.