Cressida Dick's tenure at the head of Scotland Yard begins on Monday when she formally takes charge of the country's largest police force.
Her first major public engagement will be to attend the funeral of Pc Keith Palmer, who was stabbed to death in the Westminster terror attack last month.
Ms Dick was named as the first female commissioner in the Metropolitan Police's 188-year history in February.
Last week it was revealed she will earn £40,000 less than her predecessor after opting to take an annual salary of £230,000 rather than the £270,648 she was offered.
Ms Dick was widely seen as the frontrunner to succeed Sir Bernard Hogan-Howe as commissioner after his retirement was announced last year.
The 56-year-old returns to the force she left for a low-profile role at the Foreign Office in 2015.
She will lead an organisation with more than 43,000 officers and staff and a budget in excess of £3 billion.
Ms Dick is starting in the post at a time when the Met is on high alert following Khalid Masood's deadly assault in Westminster on March 22.
Pc Palmer and four other victims were killed in the atrocity.
Masood, 52, was shot dead by armed police.
Scotland Yard plays a key role in the national counter-terrorism effort, which has seen 13 potential attacks foiled in less than four years.
As well as terrorism, Ms Dick will encounter a changing criminal landscape with the scale of fraud and cyber crime only just becoming clear.
There have also been warnings that traditional offences - such as gun and knife crime - are on the rise.
Another high-profile early task for Ms Dick will be overseeing a huge security operation around US President Donald Trump's state visit later this year.
The challenges facing the new commissioner come against a backdrop of budget squeezes which have sparked warnings from both Sir Bernard and London Mayor Sadiq Khan.
Speaking when her official starting date was announced, Ms Dick said: "Terrorism is just one of the challenges we face.
"Alongside the mayor and every single one of our officers and staff, I will work tirelessly to keep our city safe."