One of his main responsibilities will be to exploit new revenue streams and push the brand into the emerging football markets of Asia and North America.
He will also have a place on the club's nine-strong board, which includes five members of the American owner Malcolm Glazer's family.
The agency is now looking to hire a new chief executive from outside Saatchis. It has suffered from a high turnover in chief executives; Daley's successor will be the fifth since 1999, when Adam Crozier departed to head up the Football Association.
However, despite the fact that the agency this week dropped out of the top ten rankings in terms of billings, as compiled by Nielsen Research for the Campaign Top 100, Daley is confident that Saatchis is in a strong position and will continue to grow following his departure.
He said: "Over the past two years, I have put together the strongest board I have ever worked with. There is no-one in the senior management team that doesn't know what the objectives and strategies are for the business."
As the chief executive, Daley was responsible for pushing the agency in a new direction by launching industry@Saatchi, a strategic consultancy arm, and Gum, a global branded content operation.
Daley will have a huge task at United. The club is under mounting pressure to raise its income following the 2005 takeover by the businessman Glazer, who wants to raise revenues by 52 per cent to £245 million by 2010.
Daley said: "The challenge is to push the club from a brand point of view and give fans, who may never actually set foot inside Old Trafford, the chance to experience being a Manchester United fan, while obviously increasing the profits.
"My experience in the industry that I have worked in for 20 years, and the direction that I was pushing Saatchis, will be invaluable as we drive Manchester United through emerging media such as mobile and the internet."