"We will rise to the challenge. As we leave the European Union we will forge a bold new positive role for ourselves in the world, and we will make Britain a country that works not for a privileged few, but for every one of us," she said outside 10 Downing Street, vacated hours earlier by David Cameron.
Cameron stepped down after Britons rejected his entreaties to stay in the EU, a decision that has set back European efforts to forge greater unity and created huge uncertainty in Britain and across the 28-nation bloc. Just over an hour after entering her new office, May began naming ministers, appointing the steady and experienced foreign minister Philip Hammond to take charge of the finance ministry. He replaces George Osborne, whose determination to balance Britain's books made him synonymous with austerity.
In a major surprise, May named former London major Boris Johnson, a leading eurosceptic who had until recently been seen as her main rival for the prime minister's job, to take over as foreign secretary. Other prominent 'Leave' campaigners were also rewarded. One, David Davis, took the key role of Secretary of State for Exiting the European Union. Another, Liam Fox, was named to head a new international trade department.
May herself had sided with Cameron in trying to keep Britain inside the EU, so needed to reach out to the winning Leave side in order to heal divisions in the ruling party and show her commitment to respecting the popular vote. "Brexit means Brexit" has quickly become her new mantra.
By awarding such a senior job to Johnson, she also showed a conciliatory side. The two had clashed over policing in London while Johnson was serving as mayor. And since last month's vote, for which he campaigned vigorously, Johnson had suffered widespread criticism and ridicule for failing to present a clear Brexit plan and swiftly dropping out of the leadership race.
With his unkempt blonde hair, bumbling humour and penchant for Latin quotations, the man known to Britons simply as 'Boris' will be the government's most colourful figure, but a controversial choice for conducting sensitive diplomacy with world leaders. Among other appointments, rising star Amber Rudd switched from the energy ministry to take May's old job as Home Secretary. May will be Queen Elizabeth's 13th prime minister in a line that started with Winston Churchill. An official photograph showed her curtseying to the smiling monarch. She is also Britain's second female head of government after Margaret Thatcher.