Cressida Dick has been appointed as commissioner of the Metropolitan Police, replacing Sir Bernard Hogan-Howe.
The appointment means the Met will be led by a woman for the first time in its 187-year history.
Dick currently works at the Foreign Office, and her appointment had been widely expected.
London mayor Sadiq Khan welcomed the news this afternoon.
"(Cressida) has already had a long and distinguished career, and her experience and ability has shone throughout this process," he said.
"On behalf of all Londoners, I warmly welcome Cressida to the role and I very much look forward to keep[ing] our capital safe and protected."
He added: "This is a historic day for London and a proud day for me as mayor."
— Sadiq Khan (@SadiqKhan) February 22, 2017
Hogan-Howe announced his retirement from the role in September last year. He had been in the job for five years.
During his time in office, Hogan-Howe oversaw the London 2012 Olympics and dealt with the aftermath of the Paris attacks, in which the number of armed officers patrolling the streets of the capital was increased. He was also in office for the 2013 murder of Lee Rigby.