Mr Garel-Jones, 51, is seen as a mixture of Machiavelli and Ivan the Terrible by the Thatcherite right-wing.
He is one of the most colourful characters in the Government, and one of the brightest, but he never possessed the almost super-natural powers that the Thatcherites suspected he had over Baroness Thatcher.
His conspiratorial skills were put to their best in the whips' office, where he practiced the 'black arts' under successive government chief whips from 1982 to 1990. He was seen by the right as the main cause of growing 'wetness' in the Thatcher Cabinet, pushing other 'wets' such as William Waldegrave and Chris Patten for promotion.
In his defence, with justification, he insisted the left had the best candidates. In fact, Mr Garel- Jones deployed his skills to destroy rebellions by backbench Tory wets against legislation that he disliked. In the whips' office, he was meticulous in delivering government legislation and boasted: 'I would kill for Thatcher.'
He voted for Lady Thatcher in the first round of the leadership challenge by Michael Heseltine, but reserved the right to vote against her if it went to a second round. He subsequently voted for Douglas Hurd, the Foreign Secretary, although some colleagues thought he had voted for John Major because, as a foreign minister, Mr Hurd was his 'boss' at the Foreign Office.
His enemies believe there was a plot behind his support for Mr Hurd. Mr Garel-Jones had lent his Spanish home to Mr Major, with whom he shared the whips' office, and had been closer to Mr Major than Mr Hurd, but he ran Mr Hurd's hopeless campaign.
With a Spanish wife and business interests in Spain - he has a language school there - he is passionately committed to Europe. But it was for his role in bringing down Mrs Thatcher that the right cannot forgive him.
He is a founder member of the Blue Chip group of Tories from the 1979 intake; it has used his house in Westminster as its regular meeting place. It was there that members met to discuss Lady Thatcher's position after the first ballot. As a result, he became accused of hosting a plotters' meeting.
A teetotaller, but a chain-smoker, with a penchant for brown corduroy suits for Friday sittings of the House, Mr Garel-Jones was happiest in the tea-room.
He says with a clear conscience that he did not want the promotion to the Foreign Office from the whips' office. In his exchange of letters with the Prime Minister he writes: 'During the summer holidays in 1991 I asked you if, after the general election, I might be allowed to return to the back benches . . .'
During the same year, Mr Garel-Jones, a fluent Spanish speaker, interpreted for Mr Major during his visit to South America for the Earth Summit in Rio.
Born in Wales in February 1941, when his father was posted to India, he lived above his uncle's newsagent's shop in Llangennech, near Llanelli, went to the village school and chapel and spoke only Welsh until his family moved to London after the war. His father started the language school in Madrid, where Mr Garel-Jones taught for 10 years after leaving The King's School, Canterbury. He married Catalina Garrigues, the daughter of a wealthy landowner in the south of Spain, and later became a merchant banker and a name at Lloyd's - an investment that led to losses of pounds 700,000 in recent years.
Those who know him well predicted he would leave the ministerial red boxes and long nights of negotiation for the back benches. In a profile of Mr Garel-Jones, a friend was quoted in the Independent in June saying: 'One day, Tristan will just pack his bags and be off.'
That day will come at the end of the arduous passage of the Maastricht Bill, which he helped to hammer out in Maastricht. He was one of the architects, but his was more of a jobbing tradesman's role, fitting the pieces together in the political Rubik cube. First, the Dutch text had to be killed. That meant getting the other countries on Britain's side.
Like his work in the whips' office, he built the alliances then got the deal. The Thatcherites will continue to loathe him for it, but the treaty will be a testimony to his skills as a plotter.