Brown became the Prime Minister of the United Kingdom on 27 June 2007. He was succeeded by Alistair Darling as Chancellor the following day. Like all modern Prime Ministers, Brown concurrently served as the First Lord of the Treasury and the Minister for the Civil Service, and was a member of the Cabinet of the United Kingdom. Until his resignation from the post in May 2010 he was Leader of the Labour Party. He was Member of Parliament for the constituency of Kirkcaldy and Cowdenbeath until he stepped down in 2015. He was the sixth post-war Prime Minister, of a total of 13, to assume the role without having won a general election. Brown was the first Prime Minister from a Scottish constituency since the Conservative Sir Alec Douglas-Home in 1964. Not all British prime ministers have been university graduates, but, of those that were, Brown was one of only five that had not attended either Oxford or Cambridge. He proposed moving some traditional prime ministerial powers conferred by royal prerogative to the realm of Parliament, such as the power to declare war and approve appointments to senior positions. Brown wanted Parliament to gain the right to ratify treaties and have more oversight into the intelligence services. He also proposed moving some powers from Parliament to citizens, including the right to form "citizens' juries", easily petition Parliament for new laws, and rally outside Westminster. He asserted that the attorney general should not have the right to decide whether to prosecute in individual cases, such as in the loans for peerages scandal.
He said he wanted to release more land and ease access to ownership with shared equity schemes. He backed a proposal to build new eco-towns, each housing between 10,000 and 20,000 home-owners – up to 100,000 new homes in total. Brown also said he wanted to have doctors' surgeries open at the weekends, and GPs on call in the evenings. Doctors were given the right of opting out of out-of-hours care in 2007, under a controversial pay deal, signed by then-Health Secretary John Reid, which awarded them a 22 percent pay rise in 2006. Brown also stated in the manifesto that the NHS was his top priority.
On 5 June 2007, just three weeks before he was due to take the post of Prime Minister, Brown made a speech promising "British Jobs for British workers". Brown reiterated that promise at the Labour Party's annual conference in September, which caused controversy as he coupled this with a commitment to crack down on migrant workers. The Conservative Party, led by David Cameron, promptly pointed out that such a commitment was illegal under EU law.
Brown was committed to the Iraq War, but said in a speech in June 2007 that he would "learn the lessons" from the mistakes made in Iraq. Brown said in a letter published on 17 March 2008 that the United Kingdom would hold an inquiry into the war.
Brown went to great lengths to empathise with those who lost family members in the Iraq and Afghanistan conflicts. He has often said "War is tragic", echoing Blair's quote, "War is horrible". Nonetheless, in November 2007 Brown was accused by some senior military figures of not adhering to the Military Covenant, a convention within British politics ensuring adequate safeguards, rewards and compensation for military personnel who risk their lives in obedience to orders derived from the policy of the elected government.
In a speech in July 2007, Brown clarified his position regarding Britain's relationship with the USA "We will not allow people to separate us from the United States of America in dealing with the common challenges that we face around the world. I think people have got to remember that the special relationship between a British prime minister and an American president is built on the things that we share, the same enduring values about the importance of liberty, opportunity, the dignity of the individual. I will continue to work, as Tony Blair did, very closely with the American administration."
Brown and the Labour party had pledged to allow a referendum on the EU Treaty of Lisbon. On 13 December 2007, Foreign Secretary David Miliband attended for the Prime Minister at the official signing ceremony in Lisbon of the EU Reform Treaty. Brown's opponents on both sides of the House, and in the press, suggested that ratification by Parliament was not enough and that a referendum should also be held. Labour's 2005 manifesto had pledged to give the British public a referendum on the original EU Constitution. Brown argued that the Treaty significantly differed from the Constitution, and as such did not require a referendum. He also responded with plans for a lengthy debate on the topic, and stated that he believed the document to be too complex to be decided by referendum.