Tuesday marks the final full day of Cameron's six-year run as prime minister, which has come to a quicker-than-expected end after his ruling Conservative Party moved swiftly to settle on a successor.
May's rise to the country's top job will make her Britain's second female prime minister -- the first in 26 years -- and bring to an end a remarkable period of political uncertainty in the wake of Britain's shocking vote to leave the European Union on June 23.
Until Monday, Cameron had been expected to remain in office for another two months, while the two Conservative candidates vying to replace him campaigned for the votes of the party's 150,000-strong membership.
But when Energy Minister Andrea Leadsom unexpectedly pulled out of the race Monday, leaving only May, the home secretary, still in the running, Cameron announced that he would swiftly make way for the incoming leader.
"We now don't need to have a prolonged period of transition," he told reporters Monday outside 10 Downing Street, the prime minister's office and residence.
Cameron said he would resign Wednesday after attending his final prime minister's question session in Parliament, meaning May -- who was officially named leader of the Conservative Party Monday -- would be the new prime minister by Wednesday evening.
It leaves May with a drastically shortened window to decide on a leadership team. The new prime minister will face unique challenges: navigating Britain through the uncharted territory of leaving the European Union, and attempting to unite a party divided by the EU referendum and its fallout.
"It's all going to have to be a very, very careful balancing act," said CNN political contributor Robin Oakley.
May said Monday that her priorities including uniting the country and creating a "strong, new, positive vision for the future" that included not just the privileged few.
Britain has faced a leadership vacuum in the wake of the momentous vote to leave the EU, sending the pound to a 31-year low and triggering a slump in markets. Having led the campaign to remain in the union, Cameron announced his intention to resign the day that the results were announced.
One by one, figures within his party who had campaigned for a Brexit and were seen as potential successors to Cameron were knocked out of the running: MP Boris Johnson, Justice Secretary Michael Gove and Leadsom.
May, a known Euroskeptic who nonetheless backed the Remain campaign, may be an unlikely candidate to lead Britain through the divorce with Europe, but she enjoys broad party support.
She reiterated her commitment to Brexit on Monday.
"Brexit means Brexit, and we're going to make a success of it," she said.
"There will be no attempts to remain inside the EU. No attempts to rejoin it by the back door."
The pound has rallied against the dollar in the wake of the news that May would become prime minister, bringing a measure of political stability sooner than expected.
Adam Marshall, acting director general of the British Chambers of Commerce, said British business welcomed the move toward greater stability.