Montagu was born in London, and inherited his peerage in 1929 at the age of two, when his father John Walter Edward Douglas-Scott-Montagu, 2nd Baron Montagu of Beaulieu was killed in an accident. He attended St. Peter's Court School and Ridley College in Canada, Eton College and New College, Oxford. He served in the Grenadier Guards, including service in Palestine before the end of the British Mandate.
On coming of age, Montagu immediately took his seat in the House of Lords and swiftly made his maiden speech on the subject of Palestine.
Montagu's interest in historic cars led him to open the National Motor Museum on the grounds of his stately home, Palace House, Beaulieu, Hampshire in 1952.
Montagu founded The Veteran And Vintage Magazine in 1956 and continued to develop the museum, making a name for himself in tourism. He was chairman of the Historic Houses Association from 1973 to 1978 and chairman of English Heritage from 1984 to 1992.
In the 1999 reform of the House of Lords, Lord Montagu was one of 92 hereditary peers selected to remain in Parliament pending further reform of the upper chamber.
Montagu knew from an early stage of life that he was bisexual, and while attending Oxford was relieved to find others with similar feelings. In a 2000 interview Lord Montagu stated, "My attraction to both sexes neither changed nor diminished at university and it was comforting to find that I was not the only person faced with such a predicament. I agonised less than my contemporaries, for I was reconciled to my bisexuality, but I was still nervous about being exposed."
Despite keeping his homosexual affairs discreet and out of the public eye, in the mid-1950s Montagu became "one of the most notorious public figures of his generation," after his conviction and imprisonment for "conspiracy to incite certain male persons to commit serious offences with male persons," a charge which was also used in the Oscar Wilde trials in 1895, and remained on the books until 1967, and in a modified form until 2003.
In the cold war atmosphere of the 1950s, when witch hunts later called the Lavender Scare were ruining the lives of many gay men and lesbian women in the United States, the parallel political atmosphere in Britain was virulently anti-homosexual. The then Home Secretary, Sir David Maxwell Fyfe, had promised "a new drive against male vice" that would "rid England of this plague." As many as 1,000 men were locked up in Britain's prisons every year amid a widespread police clampdown on homosexual offences. Undercover officers acting as "agents provocateurs" would pose as gay men soliciting in public places. The prevailing mood was one of barely concealed paranoia.
On two occasions Montagu was charged and committed for trial at Winchester Assizes, firstly in 1953 for allegedly taking sexual advantage of a 14-year-old Boy Scout at his beach hut on the Solent, a charge he has always denied. When prosecutors failed to achieve a conviction, in what Montagu has characterized as a "witch hunt" to secure a high-profile conviction, he was arrested again in 1954 and charged with performing "gross offences" with an RAF serviceman during a weekend party at the beach hut, located on Lord Montagu's country estate.
Montagu has always maintained he was innocent of this charge as well ("We had some drinks, we danced, we kissed, that's all.") Nevertheless, Montagu was imprisoned for twelve months for "consensual homosexual offences" along with Michael Pitt-Rivers and Peter Wildeblood.
Unlike the other defendants in the trial, Montagu continued to protest his innocence. The trial caused a backlash of opinion among some politicians and church leaders that led to the setting up of the Wolfenden Committee, which in its 1957 report recommended the decriminalization of homosexual activity in private between two adults. Ten years later, Parliament finally carried out the recommendation, a huge turning point in gay history in Britain, where male homosexuality had been completely outlawed in statute law since 1533.
In a 2007 interview, when asked if he felt that he and his co-defendants had been instrumental in the decriminalization of homosexuality in Britain, Lord Montagu said,
"I am slightly proud that the law has been changed to the benefit of so many people. I would like to think that I would get some credit for that. Maybe I'm being very boastful about it but I think because of the way we behaved and conducted our lives afterwards, because we didn't sell our stories, we just returned quietly to our lives, I think that had a big effect on public opinion."
In 1958, he married Belinda Crossley, by whom he had a son and a daughter, and in 1974 he married his second wife, Fiona, and a son (Jonathan Deane) was born in 1975.
For nearly half a century, Lord Montagu steadfastly refused to speak publicly about the conviction. Montagu focused his energies instead on the National Motor Museum and other activities. But in 2000, Montagu finally broke his silence with the publication of his memoirs, Wheels Within Wheels, of which two chapters are devoted to the story of his trial and imprisonment. In interviews, Lord Montagu has stated that by publishing his story, he wanted to "put the record straight" because "I felt it was important to get it accurate."
The story of Lord Montagu's trial is told in a 2007 Channel 4 documentary, A Very British Sex Scandal.