Johnny Cartwright was a part of the West Ham United Academy and youth team and played in the 1958-59 FA Youth Cup Final team, under the stewardship of Ted Fenton. It was through Fenton that Cartwright got into coaching at an early age, whilst still playing as Fenton encouraged all the players at the Academy to take their coaching badges so that after playing they had something to fall back on. He made his senior debut for West Ham in a 1-0 win against Everton at Goodison Park on 17 October 1959.

He made four Football League for West Hame between 1959 and 1961 before signing for Crystal Palace, where he made a 12 League appearances. He then dropped into the Southern League to play for Bath City.

Johnny became the first player Wimbledon paid a fee for after they turned professional in 1964 and was considered the schemer of the forward line for several seasons, before he was side-lined by the arrival of Eddie Bailham and soon moved on to Cambridge City.

Already a fully qualified F.A. coach and a games instructor for the Greater London Council, after retiring as a player he became a coach, and has worked as academy director at Crystal Palace, youth coach at England's U18s, technical director at the English PFA and FA National School of Excellence Lilleshall and first team coach at Arsenal.