John Kerry brandished a copy of his first diplomatic passport, issued at age 11, and joked whether a man could run the State Department, winning a warm welcome on his first day as secretary of state.
Kerry was greeted by hundreds of cheering staff as he took up the reins of US diplomacy from Hillary Clinton, and joked he has "big heels to fill."
And after the recent losses of American diplomats, killed in an assault by militants on the US mission in Benghazi, Libya, Kerry promised to make the security of the department's 70,000 staff his top focus.
The "foreign service is my genes and everything that we do here is," said Kerry, the son of a US diplomat and a long-time member of the Senate Foreign Relations committee, standing among the State Department's marbled columns.
Kerry said he understood "how critical it is that you have somebody there advocating for you. The dangers could not be more clear."
He vowed that he would honor the deaths of ambassador Chris Stevens and the other Americans who died in the September 11 attack in Benghazi.
"I pledge to you this: I will not let their patriotism and their bravery be obscured by politics, number one," he said.
"Number two, 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 added, winning huge applause.
But he also showed a lighter side, wondering if after Clinton and her predecessors Condoleezza Rice and Madeleine Albright, "Can a man actually run the State Department?"
And he literally brandished his credentials for the job, taking from his pocket a battered, green copy of his old diplomatic passport, issued when he was just 11 years old.
"Number 2927...There weren't a lot of people then," he joked.
The first stamp dated back to 1954 from the French port of Le Havre, when after six days at sea first class he and his family arrived in Europe on their way to Berlin where Kerry's father was taking up a diplomatic post.
Kerry revealed how he learnt his first lessons about "the virtue of freedom" during bicycle trips around the east, Russian-held sector of the divided German city in the wake of World War II.
"That was a great adventure, and I will tell you, 57 years later, today, this is another great adventure," he added.