On 6 May 2010, May was re-elected MP for Maidenhead with an increased majority of 16,769 – 60% of the vote. This followed an earlier failed attempt by the Liberal Democrats to unseat her in 2005, as one of that party's leading "decapitation-strategy" targets.
On 12 May 2010, when May was appointed Home Secretary and Minister for Women and Equality by Prime Minister David Cameron as part of his first Cabinet, she became the fourth woman to hold one of the British Great Offices of State, after Margaret Thatcher (Prime Minister), Margaret Beckett (Foreign Secretary) and Jacqui Smith (Home Secretary). As Home Secretary, May was also a member of the National Security Council. She was the longest-serving Home Secretary for over 60 years, since James Chuter Ede who served over six years and two months from August 1945 to October 1951. May's appointment as Home Secretary was somewhat unexpected, with Chris Grayling having served as shadow Home Secretary in opposition.
May's debut as Home Secretary involved overturning several of the previous Labour government's measures on data collection and surveillance in England and Wales. By way of a government bill which became the Identity Documents Act 2010, she brought about the abolition of the Labour government's National Identity Card and database scheme and reformed the regulations on the retention of DNA samples for suspects and controls on the use of CCTV cameras. In May 2010, May announced the adjournment of the deportation to the United States of alleged computer hacker Gary McKinnon. She also suspended the registration scheme for carers of children and vulnerable people, with May saying that the measures were "draconian. You were assumed to be guilty until you were proven innocent, and told you were able to work with children." On 4 August 2010, it was reported that May was scrapping the former Labour government's proposed "go orders" scheme to protect women from domestic violence by banning abusers from the victim's home.
In June 2010, May faced her first major national security incident as Home Secretary with the Cumbria shootings. She delivered her first major speech in the House of Commons as Home Secretary in a statement on this incident, later visiting the victims with the Prime Minister. Also in June 2010, May banned the Indian Muslim preacher Zakir Naik from entering the United Kingdom.
Speaking at the Association of Chief Police Officers (ACPO) conference in June 2010, May announced radical cuts to the Home Office budget, likely to lead to a reduction in police numbers. In July 2010, May presented the House of Commons with proposals for a fundamental review of the previous Labour government's security and counter-terrorism legislation, including "stop and search" powers, and her intention to review the 28-day limit on detaining terrorist suspects without charge.
In July 2010, May announced a package of reforms to policing in England and Wales in the House of Commons. The previous Labour Government's central crime agency, Soca (Serious Organised Crime Agency), was to be replaced by a new National Crime Agency. In common with the Conservative Party 2010 general election manifesto's flagship proposal for a "Big Society" based on voluntary action, May also proposed increasing the role of civilian "reservists" for crime control. The reforms were rejected by the Opposition Labour Party.
On 9 December 2010, in the wake of violent student demonstrations in central London against increases to higher-education tuition fees, May praised the actions of the police in controlling the demonstrations but was described by The Daily Telegraph as "under growing political pressure" due to her handling of the protests.
In December 2010, May declared that deployment of water cannon by police forces in mainland Britain was an operational decision which had been "resisted until now by senior police officers." She rejected their use following the widespread rioting in summer 2011 and said: "the way we police in Britain is not through use of water cannon. The way we police in Britain is through consent of communities." May said: "I condemn utterly the violence in Tottenham... Such disregard for public safety and property will not be tolerated, and the Metropolitan Police have my full support in restoring order."
In July 2010, May proposed to review the previous Labour Government's anti-social behaviour legislation signalling the abolition of the "Anti-Social Behaviour Order" (ASBO). She identified the policy's high level of failure with almost half of ASBOs breached between 2000 and 2008, leading to "fast-track" criminal convictions. May proposed a less punitive, community-based approach to tackling social disorder. May suggested that anti-social behaviour policy "must be turned on its head", reversing the ASBO's role as the flagship crime control policy legislation under Labour. Former Labour Home Secretaries David Blunkett (who introduced ASBOs) and Alan Johnson expressed their disapproval of the proposals.
In 2010, May promised to bring the level of net migration down to less than 100,000. The Independent reported in February 2015, "The Office for National Statistics (ONS) announced a net flow of 298,000 migrants to the UK in the 12 months to September 2014—up from 210,000 in the previous year." In total, 624,000 people migrated to the UK in the year ending September 2014 and 327,000 left in the same period. Statistics showed "significant increases in migration among both non-EU citizens—up 49,000 to 292,000—and EU citizens, which rose by 43,000 to 251,000."
On 2 July 2010, May stated she would be supporting the previous Labour Government's Anti-Discrimination Laws enshrined in the Equality Act 2010 despite having previously opposed it. The Equality Act came into effect in England, Wales and Scotland on 1 October 2010. She did however announce that a clause she dubbed "Harman's Law" which would have required public bodies to consider how they can reduce socio-economic inequalities when making decisions about spending and services would be scrapped on the grounds that it was "unworkable".