By turns emotional and feisty, Clinton told U.S. lawmakers that she took responsibility for the incident in which U.S. Ambassador Christopher Stevens and three other Americans were killed, but also defended herself.
She interrupted one senator to contradict him and stressed that she did not see requests for additional security to protect the Benghazi mission.
Clinton cast the incident as part of a long history of such violence as well as the result of regional instability since the Arab Spring of popular revolutions began in 2011.
Senator Bob Corker, a senior Republican, kept up the criticism of the Obama administration, saying the Benghazi attack and the U.S. response displayed "woeful unpreparedness" for the events sweeping the region.
Speaking in congressional testimony delayed by more than a month because of her ill health, Clinton's voice cracked as she spoke of comforting families who lost loved ones in the attack, the first since 1988 in which a U.S. ambassador was killed.
"For me, this is not just a matter of policy - it's personal," Clinton told the Senate Foreign Relations Committee on what is likely to be the last day that she will testify before Congress before stepping down as secretary of state.
"I stood next to President (Barack) Obama as the Marines carried those flag-draped caskets off the plane at Andrews," she added, her voice breaking as she described the ceremony at Andrews Air Force Base when the men's remains were brought home.
"I put my arms around the mothers and fathers, the sisters and brothers, the sons and daughters and the wives left alone to raise their children," she said.
The attack could haunt Clinton - who is expected to step down in the coming days once her designated successor, Senator John Kerry, is confirmed by the U.S. Senate - should she decide to run again for president in 2016, a possibility she has played down.
Militants attacked and overwhelmed the U.S. diplomatic facility in Benghazi, Libya, on September 11 in a sustained assault.
The official inquiry concluded that the State Department was completely unprepared to deal with the attack, citing "leadership and management" deficiencies, poor coordination and unclear lines of authority in Washington. The inquiry did not find Clinton personally at fault.
'I TAKE RESPONSIBILITY'
"I take responsibility," Clinton said, echoing comments she first made in a TV interview on October 15 and stressing that she has accepted all of the recommendations of an independent review panel that ultimately held lower-level officials responsible.
"Nobody is more committed to getting this right. I am determined to leave the State Department and our country safer, stronger and more secure," she added.
Four State Department officials were put on administrative leave following the board's report.
A separate Senate committee report said the State Department made a "grievous mistake" in keeping the Benghazi mission open despite inadequate security and increasingly alarming threat assessments in the weeks before the attack.
Clinton was due to testify later in the day before a House of Representatives committee.
Clinton was originally due to appear before the committee on December 20 but had to cancel after she suffered a concussion when she fainted due to dehydration. Doctors later found she had a blood clot in her head and hospitalized her for several days.
The controversy over the Benghazi attack also cost Susan Rice, the U.S. ambassador to the United Nations, her chance to succeed Clinton as secretary of state.
Republicans in Congress harshly criticized Rice for her comments days after the attack in which she said the incident appeared to be the result of a spontaneous demonstration rather than a planned assault. Rice eventually withdrew her name from consideration for the top U.S. diplomatic job.
Clinton implicitly sought to defend herself against similar attacks, saying that "the very next morning I told the American people that 'heavily armed militants assaulted our compound' and vowed to bring them to justice."
(Additional reporting by Patricia Zengerle and Tabassum Zakaria; Editing by Will Dunham)