GOP lawmakers repeatedly questioned Clinton about whether she had seen earlier requests for beefed-up security.
"I did not see these requests. They did not come to me. I did not approve them. I did not deny them," she said.
That provoked a testy response from Sen. Rand Paul, R-Ky., a potential presidential candidate in 2016. He excoriated Clinton and expressed disbelief that she hadn't read the cables about security concerns.
"Had I been president at the time and I found that you did not read the cables from Benghazi, you did not read the cables from Ambassador Stevens, I would have relieved you of your post," Paul told Clinton. "I think it's inexcusable.
Clinton took Republicans to task, chiding House GOP members for recently stripping $1 billion in security aid from the hurricane relief bill and the Senate panel for failing for years to produce an authorization bill.
In northern California, Stevens' stepfather, Bob Commanday, said the family has avoided discussions of whether security was adequate. He said Clinton had been in contact with the family on several occasions since the attack.
"We're very aware of her sympathy because of our contact with her and the way she has connected with us and written to us," he said. "It's a tragedy and nothing that is said or done can bring him back, so we are just going on with life."
In something of a valedictory, Clinton noted her robust itinerary in four years and her work, nearly 1 million miles and 112 countries.
"My faith in our country and our future is stronger than ever. Every time that blue and white airplane carrying the words "United States of America" touches down in some far-off capital, I feel again the honor it is to represent the world's indispensable nation. And I am confident that, with your help, we will continue to keep the United States safe, strong, and exceptional."
Clinton was the sole witness at back-to-back hearings before the Senate and House foreign policy panels on the September raid. She had been scheduled to testify before Congress last month, but an illness, a concussion and a blood clot near her brain forced her to postpone her appearance.
Absent from the hearing was Kerry. Sen. Bob Menendez, D-N.J., the incoming chairman, presided over the hearing.
Clinton's testimony was focusing on the Libya attack after more than three months of Republican charges that the Obama administration ignored signs of a deteriorating security situation there and cast an act of terrorism as mere protests over an anti-Muslim video in the heat of a presidential election. Washington officials suspect that militants linked to al-Qaida carried out the attack.
Politics play an outsized role in any appearance by Clinton, who sought the Democratic presidential nomination in 2008 and is the subject of constant speculation about a possible bid in 2016. The former first lady and New York senator — a polarizing figure dogged by controversy — is about to end her four-year tenure at the State Department with high favorable ratings.
A poll early last month by the Pew Research Center for the People & the Press found 65 percent of Americans held a favorable impression of Clinton, compared with 29 percent unfavorable.
On the panel at the hearing were two possible 2016 Republican presidential candidates — Florida's Marco Rubio and Paul, a new member of the committee.
Clinton did little to quiet the presidential chatter earlier this month when she returned to work at the State Department after her illness. On the subject of retirement, she said, "I don't know if that is a word I would use, but certainly stepping off the very fast track for a little while."
Associated Press writers Bradley Klapper and Andrew Miga in Washington, and Garance Burke in San Francisco contributed to this report.