Montgomerie was born into an army family in Barnstaple in 1970. He said in a Guardian interview that “his teenage Thatcherism was tempered by discovering evangelical Christianity at sixteen”.

At Exeter University, Montgomerie and Burrowes also started the Conservative Christian Fellowship (CCF) in December 1990, supported by the Christian Coalition of America. During this period, he argued that the Conservative Party should form closer links with churches on issues such as homosexuality and Section 28, saying that the party should “expose the unbiblical and the libertine”. He has since reversed his position on those issues. He served as Director of the CCF from 1990 to 2003.

From 1998 to 2003, Montgomerie was the speech-writer for two Conservative Party leaders, William Hague, and then Iain Duncan Smith. He also had responsibility for the Conservative Party’s outreach to faith communities and the voluntary sector. In September 2003, Montgomerie became Conservative Party leader Duncan Smith’s Chief of Staff; Duncan Smith was replaced by Michael Howard two months later. He had become a main influence behind Duncan Smith’s theme of compassionate conservatism.

In 2004, with Iain Duncan Smith and Philippa Stroud, Montgomerie established the Centre for Social Justice to take forward the work on “compassionate conservatism” that Smith had begun as party leader. Following the tradition of people such as William Wilberforce, the Earl of Shaftesbury and Richard Oastler he aimed to make the condition of the poor a priority. He established a social action project called “Renewing One Nation” which helped Duncan Smith focus on these issues.

On 28 March 2005, Montgomerie launched the ConservativeHome website in the period just before the general election campaign that year. With Conservative MP John Hayes, he also set up conservativedemocracy.com, which successfully co-ordinated grassroots opposition to party leader Michael Howard’s attempt to abolish the “one member, one vote” rule in the 2005 Conservative leadership election.

In September 2006, The Independent described Montgomerie as “emerging as a major player in Tory politics.” He was critical of the A-List and argued that the party leader after 2005, David Cameron, was in danger of alienating working-class Tory voters, and pressed Cameron for specific pledges on tax cuts. He supported the introduction of same-sex marriage in England and Wales arguing that it was a way to strengthen the institution more generally. Through ConservativeHome, Montgomerie was used as an expert on internet campaigning by Conservative Central Office.

Montgomerie was a director of the internet television channel 18 Doughty Street which began broadcasting in October 2006 and went off air in November 2007. While at 18 Doughty Street, Montgomerie hosted its ‘Campaign HQ’ programme, which developed the channel’s Internet political advertisements after allowing viewers to vote on a choice of (usually) three different proposals. Previous adverts included attacks on taxes, state funding of political parties, and London Mayor Ken Livingstone. The latest, “A World Without America”, with an end scene depicting the Statue of Liberty wearing a burqa, was co-produced by 18 Doughty Street and the website BritainAndAmerica, and had 50,000 views within its first 24 hours of publication.

Through his prominence with ConservativeHome, Montgomerie wrote frequent articles on Conservative politics for The Guardian and The Times, and occasionally for the Daily Mail, The Independent, and the Financial Times. In April 2011, he became a columnist for The Sunday Telegraph, but in October of the same year, Montgomerie resigned from his column, after a series of attacks on him by the Mandrake column in The Daily Telegraph, its sister paper. Montgomerie wrote that a tweet critical of the Daily Telegraph editor Tony Gallagher had started the attacks: “A senior Telegraph journalist told me that Tony Gallagher… was “sh*tbagging” me for daring to tweet a criticism of the newspaper last week.” Montgomerie became a columnist for The Times soon after.

Montgomerie continued to edit ConservativeHome alongside others including co-editor Jonathan Isaby, assistant editor Joseph Willits, deputy editor Matthew Barrett, and Isaby’s replacement, former Conservative MP Paul Goodman After the 2010 general election Montgomerie wrote a report that was critical of David Cameron’s election campaign, entitled “Falling short”. The Guardian listed him at number 81 in their 2012 list of the most powerful people in the media.

Montgomerie has promoted the Cultural Marxism conspiracy theory and wrote in The Times in 2013 “The 20th century was far from an overwhelming victory for the right. Though revolutionary Marxism died, its fellow traveller, cultural Marxism, prospered.”

In February 2013, Montgomerie announced that in April that year he would join The Times as comment editor, replacing Anne Spackman, but maintained a role as an “advisor” and weekly blogger for ConservativeHome.

He founded the UnHerd website in 2017 “to appeal to people who instinctively refuse to follow the herd and also… to investigate ‘unheard’ ideas, individuals and communities”. On 25 September 2018, he announced that he had left UnHerd.

In September 2019, Montgomerie was appointed as “social justice adviser” to the Prime Minister in Number 10 Downing Street. On 31 January 2020, he said on the BBC’s Politics Live that the role lasted until the election was called in November 2019 and that he was now in discussions with Boris Johnson regarding a new advisory role following Johnson’s success in that election. By May 2020 Montgomerie had become a frequent critic of the government, in particular of the Prime Minister’s Senior Adviser Dominic Cummings and the Prime Minister himself.

In 2019, Montgomerie was reported as saying that the British government should have a “special relationship” with Hungary post-Brexit, saying at a meeting of the Danube Institute that “Budapest and Hungary have been home, I think, for an awful lot of interesting early thinking on the limits of liberalism, and I think we are seeing that in the UK as well. So I hope there will be a special relationship with Hungary amongst other states.”