As part of the new deal, the ceremony will move in its 15th anniversary year to the O2 arena in London, and will also move from its traditional autumn slot to January 2010.
"I'm delighted to announce this new deal which secures the NTAs - the most popular television awards, as chosen by viewers - on ITV1 until 2011 at a fantastic new home in the O2 arena," said ITV's Elaine Bedell.
"I'm also very pleased to be announcing Dermot as the new host of the awards, and I have no doubt that he's the right man to continue the excellent job that Sir Trevor has done over the last 13 years."
New host O'Leary added: "The NTAs have always been one of the biggest nights in British television and what makes it extra special is the fact that all the winners are 100% voted for by the viewers.
"Taking over from Sir Trevor is a daunting prospect but it's a genuine honour and I'm really looking forward to it. I'm especially looking forward to compering the EastEnders/Coronation Street blood feud. May the best soap win!"
McDonald said that after 13 years, it is "the right time" for him to relinquish his role.