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 is the sole witness at back-to-back hearings before the Senate and House foreign policy panels on the September raid.
Clinton 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 Sen. John Kerry, D-Mass., the man tapped to succeed Clinton. His swift Senate confirmation is widely expected. Kerry's confirmation hearing is scheduled for Thursday.
Clinton's testimony will focus on the attack after more than three months of Republican charges that the Obama administration ignored signs of a deteriorating security situation in Libya 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.
"It's been a cover-up from the beginning," Sen. John McCain, R-Ariz., the newest member of the Senate Foreign Relations Committee, said Tuesday.
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.