HILLARY Clinton has formally resigned as the 67th US secretary of state, capping a four-year tenure in the office during which she shattered previous records for the number of countries visited.
In a letter sent to the President, Barack Obama, on Friday just before she was to leave the State Department for the last time in her official capacity, Mrs Clinton thanked her former foe for the 2008 Democratic presidential nomination for the opportunity to serve in his administration.
Mrs Clinton said it had been an honour to be part of his cabinet and that she remained convinced of the ''strength and staying power'' of American global leadership.
''I am more convinced than ever in the strength and staying power of America's global leadership and our capacity to be a force for good in the world,'' she said in the letter.
Her resignation will be effective on the swearing-in of her successor, John Kerry, who was to take the oath of office in a private ceremony on Friday in Washington.
Mrs Clinton left office with a slap at critics of the Obama administration's handling of the September attack on a US diplomatic mission in Libya.
She said in an interview on Thursday that critics of the administration's handling of the attack did not live in an ''evidence-based world,'' and their refusal to ''accept the facts'' was unfortunate and regrettable for the political system.
Mrs Clinton said the attack in Benghazi was the low point of her time as America's top diplomat. But she suggested that the furore over the assault would not affect whether she would run for president in 2016.
She said she ''absolutely'' still planned to make a difference on issues she cared about in speeches and in a sequel to her 2003 memoir, Living History, that will focus largely on her years as secretary of state.
Mrs Clinton spoke in her outer office on the seventh floor of the State Department less than 24 hours before she walked out for a final time as boss. She was relaxed but clearly perturbed by allegations from Republican lawmakers and commentators that the administration had intentionally misled the public about whether the attack was a protest gone awry or a terrorist attack, or intentionally withheld additional security for diplomatic personnel in Libya knowing that an attack could happen.
An independent panel she convened to look into the incident was scathing in its criticism of the State Department and singled out four officials for serious management and leadership failures.
But it also determined there was no guarantee that extra personnel could have prevented the deaths of the US ambassador to Libya, Chris Stevens, and three other Americans.
Mrs Clinton was not blamed, although she has said she accepted responsibility for the situation.
''I was so unhappy with the way that some people refused to accept the facts, refused to accept the findings of an independent Accountability Review Board, politicised everything about this terrible attack,'' she said. ''My job is to admit that we have to make improvements and we're going to.''
Hours after Mrs Clinton made those remarks, a suicide bomber linked to a domestic terror group exploded a device just outside the US embassy in Ankara, Turkey, killing himself and a guard.
Mrs Clinton is no stranger to partisan politics. As first lady, she railed in 1998 against a ''vast right-wing conspiracy'' that she asserted had been attacking her husband, Bill Clinton, ever since he had become president.
But the woman who was once considered a divisive figure in American politics, yet leaves office as one of its most popular, remained coy on whether she would run for president in 2016.
''I am making no decisions, but I would never give that advice to someone that I wouldn't take myself,'' she said. ''If you believe you can make a difference, not just in politics, in public service … then you have to be prepared to accept that you are not going to get 100 per cent approval.''