Despite interest from a top four top-flight club, Harding was the subject of a late bid at the weekend but rejected a lucrative offer, he flew back to England from a family holiday in the States to confirm his switch to Leeds.
Harding is out of contract at the Withdean Stadium but will command a fee under FIFA laws because of his age, and Brighton's board were today considering an offer from Ken Bates.
Albion boss Mark McGhee last night claimed he had been left in the dark over United's agreement, as he commented: "I'm very disappointed, not just for me but for the chairman and everyone else at the club who has looked after him. Most of what we are hearing is through the newspapers and second and third hand. I totally understand his motivation. He wants to earn more money but I think he should have treated us a bit better.
"I'm also very disappointed in Kevin Blackwell because he has not phoned me to say they are talking to him or that they want him. That's not the way I would do things but I suppose that's the game we are in."
Harding was Brighton's left-back throughout the 2004-05 season but is also suited to the left wing and this may allow Blackwell to cure a long-standing situation.
David Healy played out wide for much of the last campaign but Harding's arrival could give him the chance at last to form a regular partnership with Rob Hulse up front.