May, 59, was left as the only candidate to succeed David Cameron, who announced he was stepping down after Britons voted last month to leave the European Union.
Britain's planned withdrawal has weakened the 28-nation bloc and created huge uncertainty over trade and investment.
May and energy minister Andrea Leadsom had been due to contest a ballot of around 150,000 Conservative party members, with the result to be declared by September 9.
But Leadsom unexpectedly withdrew on Monday, opening the way for May to take over much sooner.
Her victory means that the complex process of extricating Britain from the EU will be led by someone who favoured a vote to Remain in last month's membership referendum.
May, who has served as interior minister for the past six years, is now set to become Britain's second female prime minister after Thatcher, although it was not clear exactly how soon that would happen.
