Mr Kenny's preferred candidate, Pat Cox, was eliminated after the first count after receiving less than 25 per cent of the vote. Mr Mitchell comfortably won on the second count, 55 per cent to 45 per cent, ahead of Mairead McGuinness, at the convention held at the Regency Hotel, Dublin. The result was announced shortly after 5pm, somewhat earlier than had been anticipated.
A clearly unimpressed Mr Kenny spoke sternly to Mr Mitchell after a press conference, during which Mr Mitchell revealed he was to take a week's holidays.
Making clear his displeasure to Mr Mitchell in front of reporters, Mr Kenny said: "Take your holidays and enjoy it because, believe you me, when you come back you better be ready for one hell of a campaign."
Mr Mitchell, however, took his own swipe at the party hierarchy over their lack of support for his candidacy and their support for Mr Cox, declaring that the real party hierarchy won out.
"We are the hierarchy, the parliamentary party; the executive council made a very firm point. They re-established themselves and made a very firm point as to who makes the decisions," he said.
Moments later, a sullen Taoiseach was seen in deep conversation with Fine Gael General Secretary Tom Curran.
When challenged by a reporter at the press conference that he looked disappointed, Mr Kenny responded sharply: "Am I supposed to be going around grinning like a Chesire cat at everything?"
There was further criticism from Fianna Fail TD Niall Collins that the date of the Presidential election was revealed at a Fine Gael party event. Charlie Flanagan, who is the Fine Gael director of elections, revealed that the election was to be held on October 21.
Mr Collins said: "Using a party event to formally announce the date of the election is a disgrace and confirms Fine Gael's growing arrogance in Government."
A spokeswoman for Fine Gael later insisted that the October 21 date was not fully confirmed but was only a working date at present.
It was shortly after 5pm when Brian Murphy, chair of the Fine Gael Executive Council, made the announcement at the Regency Hotel, to huge applause from the 700 delegates.
Mr Mitchell immediately made his way to the podium to address the crowd.
"This is only the start of this, we now look to the next step in the 12 weeks ahead. Thank you all," he said.
Speaking to the Sunday Independent, a clearly disappointed Pat Cox said he wished Mr Mitchell all the best. He said he was grateful that the Fine Gael party had opened their hearts to him.
"I approached this with a sense of calm and I had to prepare myself for defeat as much as for victory. But I am on board and will help in whatever way I can," he said.
Many TDs in attendance were in agreement that Mr Cox was the best speaker on the day with Ms McGuinness's address falling short of expectations. Mr Cox had told the gathering that he was best placed to win the Presidency for Fine Gael.
Despite that, the mood of the room earlier in the day suggested Ms McGuinness was ahead. During her address, she spoke of her electoral record and stressed her record in the European Parliament. Ms McGuinness said her determination to succeed for the party was unquestioned. Mr Mitchell said he had the "heart and the history" to make the historic breakthrough in electing a Fine Gael president.
Before the result was announced, Mr Kenny said the next presidential election would be "the most crucial since the foundation of the State". He also said the Fine Gael candidate would have a "decent chance" of winning.
Tomorrow, independent election candidates David Norris, Sean Gallagher and Mary Davis will attempt to enlist the support of North Tipperary County Council. Later in the day, Ms Davis will canvass Limerick County Council for its endorsement in an attempt to consolidate support for her Aras bid within the Mid-West region.