Secretary of State John Kerry says that he has "big heels to fill" as Hillary Clinton's successor and promised to keep U.S. diplomats abroad safe.
Kerry was greeted by cheering employees at the State Department as he arrived for his first day of work on Monday.
As the first man in the post in eight years, Kerry referred to Clinton and Condoleezza Rice, his two most recent predecessors, asking: "Can a man actually run the State Department?"
"I don't know," he joked. "As the saying goes, I have big heels to fill."
Nothing the deaths of Ambassador Christopher Stevens and three other Americans in the Sept. 11, 2012, terror attack on the U.S. diplomatic post in Benghazi, Libya, Kerry said the protection of American diplomats would be one of his top priorities.
"I guarantee you that beginning this morning when I report for duty upstairs, everything I do will be focused on the security and safety of our people," he said.
He pledged not to "let their patriotism and their bravery be obscured by politics."
Kerry told agency employees that he and President Barack Obama needed their help to make America safer and the world more prosperous and peaceful.
The son of a diplomat and member of the Senate Foreign Relations Committee for 28 years said that while the Senate may be in his blood, Foreign Service and diplomacy are in his genes.
He spoke of his time as a child in Germany, where his family lived while his father was posted on assignment.
Kerry pulled out his first diplomatic passport, issued when he was 11 in 1954, recounting how he used it a year later to ride his bicycle across the divided capital of Berlin and into the Soviet-controlled eastern part of the city.
"If the tabloids today knew I had done that, I can see the headlines that say, 'Kerry's Early Communist Connections!"' he joked.
He said his parents were mad at him, telling him he could have caused a diplomatic incident for crossing the Iron Curtain.
Kerry described the experience as his first lesson in democracy.
There were very few people," he told the crowd. "There was no joy in those streets. And when I came back, I felt this remarkable sense of relief and a great lesson about the virtue of freedom and the virtue of the principles and ideals that we live by and that drive us."
Kerry was sworn into office in a private ceremony last Friday before a more formal swearing-in ceremony on Wednesday at the State Department.
He is expected to spend the remainder of the week holding meetings and reaching out to world leaders.
On Monday he spoke with the foreign ministers of Britain, France and Germany. And over the weekend he spoke with Israeli Prime Minister Benjamin Netanyahu and Palestinian President Mahmoud Abbas, as well as the foreign ministers of Japan, South Korea, Turkey, Canada and Mexico.
He also crashed a concert by the Afghan National Institute of Music on Monday. When told the group of musicians from Kabul was in the building, he went down to meet them, take photos and listen to their performance.
Calling them "ambassadors of peace," Kerry related to them as a fellow musician. As a young senator, Kerry played guitar in a rock band called the "Electras."
"I still play guitar," he told them. "When I go home, I love to play guitar."