Labour’s new leader is a stalwart of the party’s left-wing, an anti-war campaigner and serial Commons rebel who will be as surprised as anyone else to find himself thrust into the role.
Few people who have never joined rallies for peace, nuclear disarmament or Palestine would have been aware of the mildly-spoken, bearded politician despite 32 years as an MP.
But now he finds his actions past and present under the full glare of public scrutiny and facing the daunting task of uniting his parliamentary ranks behind a radical programme few of his colleagues believe can get them back into power.
As the search for someone to reinvigorate the party after May’s crushing general election began, the 66-year-old was not even the most obvious candidate to be a token “Old Labour” no-hoper.
But after being persuaded to take that role and scraping on to the ballot paper thanks only to the nominations of non-supporters keen for a “wide debate”, his emergence as a national figure was meteoric.
His policy platform is an unapologetic menu of the sorts of reforms deemed unpalatable since the creation of New Labour and the pursuit of centre-ground voters which propelled Tony Blair to three successive general election triumphs.
The railways would be renationalised, student tuition fees abolished and - most strikingly - economic austerity abandoned in favour of a “People’s Quantitative Easing” to print money to build homes and other infrastructure.
An opponent of the UK’s membership of Nato and less than enthusiastic about the European Union, many of his long-held positions are out of step with all but a handful of the party’s current crop of MPs, albeit a few are among the new arrivals.
He has been unwavering in his commitment to unfashionable causes - courting controversy by talking to and sharing platforms with the IRA, Hamas and other groups in the belief that only dialogue with both sides can end violence.
The depth of the chasm between his deeply-held beliefs and the parliamentary party he now leads is neatly illustrated by one striking number: the around 530 times he has voted against his predecessors in the Commons since 1997.
So his willingness to make the sort of compromises necessary to form a shadow cabinet and maintain any discipline behind him in the ranks will be an early - and crucial - test of his mettle.
A very private man, he will also be braced for his past to be raked through minutely for further controversy by opponents.
That past includes three marriages - the second of which foundered in part because of his then wife’s insistence that one of their three sons attend a grammar school despite his opposition to selection.
The son of an engineer and a maths teacher, he grew up in Shropshire where his own education was at the highly-rated Adams’ Grammar School and as a teenager became involved in causes such as the Campaign for Nuclear Disarmament.
Unfinished studies at North London Polytechnic were followed by election to Haringey Council in north London in 1974.
He worked on socialist icon Tony Benn’s failed bid for the Labour deputy leadership before entering the Commons as MP for Islington North in 1983, a seat he has held ever since.
One of his most consistent successes has been in the Parliamentary Beard of the Year award - with his facial hair being declared most impressive a record five times.