Mr. Biden’s decision caps an unusually rapid ascent for Ms. Khan. She was born in London to Pakistani parents who emigrated to the United States when she was 11. She first rose to prominence while a law student at Yale, where she wrote a paper laying out how modern antitrust laws had failed to check the power of Amazon. The paper attracted notice from policymakers, other lawyers and the press.
Quiet and generally averse to the spotlight, she has played a critical role behind the scenes in recent years as a senior aide to the House judiciary committee on its 16-month investigation of competition among digital platforms. She also served as a counselor to the F.T.C. commissioner Rohit Chopra. She joined the faculty at Columbia Law School last year.
Ms. Khan is replacing Rebecca Kelly Slaughter, a former Democratic Senate staff member who was named the agency’s acting chairwoman in January. Ms. Slaughter’s office said she would remain at the agency as a commissioner.
“If she chose to be a doctor, she would have been a star doctor. If she’d chosen to go to Wall Street, she’d be running a very powerful fund,” said Barry Lynn, the director of the Open Markets Institute, a think tank, and Ms. Khan’s former boss. “I am thrilled that she chose this path because she has the ability to transform America’s political economy.”
Her appointment was also hailed by many Democratic lawmakers.
Senator Amy Klobuchar, chairwoman of a Senate subcommittee overseeing antitrust and a regular critic of the tech industry, said Ms. Khan’s “deep understanding of competition policy will be vital as we strengthen antitrust enforcement.”
Ms. Klobuchar noted Ms. Khan’s new role in an afternoon hearing on competition, before any announcement from the White House.
“We need all hands on deck as we take on some of the biggest monopolies in the world,” she said.
Senator Elizabeth Warren of Massachusetts said, “With Chair Khan at the helm, we have a huge opportunity to make big, structural change by reviving antitrust enforcement and fighting monopolies that threaten our economy, our society and our democracy.”