Mr Cox, a former director of LWT who made millions from his share options when the ITV franchise was taken over, joined the ITV Association in 1995 with a brief to handle ITV's relations with the government and mastermind the network's switch to digital television.
Both those roles will now be taken over by Richard Eyre, who took over as chief executive of ITV last year, in a move which strengthens his control over the network as he tries to reverse its slipping ratings. The ITV Association will be merged with the ITV Network Centre and Mr Cox's job will disappear.
Mr Cox said the departure was "perfectly amicable" while Mr Eyre said he was "very sorry" to see Mr Cox go.
However, ITV's move into digital television has been plagued by problems, and the network is now thought likely to be one of the last analogue channels to make the switch to digital.
Mr Cox said he did not have another job lined up, and was in no hurry to move on.