His departure was widely expected after the appointment of long-time rival Greg Dyke as chairman of Pearson Television, Thames's parent company - an appointment which effectively passed over Mr Dunn for the top job.
"Two into one would not go," a Pearson insider said yesterday. "There was only one senior job in global television at Pearson, and two men who wanted it.
"The two were given a few months to see if they could come up with a modus operandi in managing together. Obviously, Richard made a measured decision that it was time to move on."
Mr Dunn was believed last night to be weighing several offers, including a senior position with SES, the Luxembourg-based owner of the Astra satellite. Sources indicated that he had at least two firm offers on the table. "There certainly will be no shortage of opportunities," a source at Thames Television said.
Mr Dunn worked on Pearson's joint venture deals with the British Broadcasting Corporation and in the planning for an investment in Hong Kong satellite television.
He built the rump of Thames Television into a profitable production company after it was out-bid by Carlton for the London weekday ITV licence in the last round of franchise awards.
Mr. Dyke, 47, left London Weekend Television last year, following the company's takeover by Granada.
He was one of 50 executives at LWT who earned a total of £70m from a controversial shares deal before the Granada takeover. He subsequently opted to take the Pearson job instead of heading a bid for Channel 5, the new terrestrial channel.
Friends of both men insist they get along reasonably well, despite the differences in styles.
It is thought that the sometimes difficult relationship between Mr Dunn and the Pearson group's chief executive Frank Barlow, who is directing the company's push into global television, was at the root of Mr Dunn's decision to leave.
Mr Dunn is described by colleagues as an urbane diplomatic man and as different from Mr Dyke as chalk and cheese.