Barnes, who is only the second sales director in Channel 4's history, has been in sole charge of the broadcaster's airtime, sponsorship and online sales departments since 1997.
In that time, he has helped deliver more than £12 billion in sales revenue for Channel 4.
Barnes will leave his post this summer, but continue to work with the broadcaster via his newly-formed sales consultancy, while also widening his non-executive portfolio.
David Abraham, the Channel 4 chief executive, described Barnes as an "outstanding sales director" for Channel 4 over the last 20 years.
"I am the last in a long line of chief executives who owe him a great deal," said Abraham.
"His massive contribution has helped the channel to become what it is today and his shoes will be big ones to fill. I, along with a multitude of colleagues and peers, wish him the very best for the future and thank him for everything he has done."
Barnes added: "I have had a fantastic time at Channel 4 and achieved some great results. I've also worked with some outstandingly bright people, both at the channel and in the agencies and clients we deal with, and they have all contributed to the success Channel 4 sales is today.
"I think David's strategy for the future is spot on and I have no doubt he will be successful in implementing it. Now, however, I will observe it from afar as opposed to being instrumental in its origination and execution.
"As you would expect, after such a long time at one place, my emotions are a mixture of exhilaration, trepidation and pride. If the next 20 years are only half as good as the previous 20 for both me and Channel 4, then I am confident we will both reach that point in the rudest of health!"
Since joining Channel 4 in 1991, Barnes has helped increase the broadcaster's share of net advertising revenue from 18% in 1993 to a record high of 24.8% last year. Former Channel 4 chief executive Michael Grade once commented that he was "equally at home in the boardroom and the bar, a rare combination".
Barnes is also currently the chairman of commercial TV marketing body Thinkbox and sits on the board of audience measurement body BARB.