Janet Yellen, the first woman to be Treasury secretary, is sworn in by the first woman to be vice president.

Janet L. Yellen was sworn in as Treasury secretary on Tuesday by Vice President Kamala Harris, a history-making moment as both women are the first to assume two of the most powerful jobs in the United States government.

Ms. Yellen is the nation’s 78th Treasury secretary and the first woman to assume that role in the institution’s 232-year history. She is also the first woman to have held all three top economic jobs in the government, having served as chair of the Federal Reserve and the Council of Economic Advisers.

She is taking on the job at a time of economic crisis, with millions still out of work and the recovery slowing as the virus persists. Ms. Yellen will quickly be thrust into fraught negotiations over how to design and pass a robust stimulus package to help revive an economy that has been hammered by the coronavirus pandemic.

Standing outside the White House, Ms. Yellen took the oath of office with her husband, the economist George Akerlof, and her son by her side. At the conclusion of the ceremony, Ms. Harris said, “Congratulations, Madam Secretary.” To which Ms. Yellen replied, “Thank you, Madam Vice President.”

In a sign of the task ahead, the Treasury Department has been rapidly adding staff and advisers in recent days. Ms. Yellen was confirmed by a bipartisan vote on Monday but her top deputy, Wally Adeyemo and other senior officials who will oversee the department’s international affairs, sanctions and domestic finance divisions are not yet in place and will require Senate confirmation.

The White House and lawmakers in Congress have begun the fraught process of negotiating over President Biden’s proposed $1.9 trillion relief bill. Ms. Yellen, a labor economist and former Federal Reserve chair, will now assume a central role in making the case for why the economy needs more fiscal support.

At her confirmation hearing last week, Ms. Yellen told senators that it was time to “act big” and that doing so would be fiscally responsible in the long run by creating a healthier economy.