It is third time lucky for John McDonnell, who twice tried to run as the left’s candidate for the Labour leadership but was kept off the ballot.

This time round, he decided not to try again himself but ended up persuading his close ally Jeremy Corbyn to stand, and helped mastermind the campaign as his agent. The role of shadow chancellor looks very much like a reward for his efforts, but Corbyn will also know the importance of having someone with absolute loyalty in this role.

A former National Union of Mineworkers and Trades Union Congress official, McDonnell’s political career started in the Greater London Council as deputy to Ken Livingstone and he went on to become MP for Hayes and Harlington in 1997.

During his time in parliament, the 64-year-old has never served as a minister or shadow minister, but like Corbyn, has made his name as a rebellious backbencher. The pair have often worked together voting against issues such as the Iraq war and anti-terror laws. He also chairs the Socialist Campaign Group of MPs.

When it came to the post-Miliband leadership contest, McDonnell

recently revealed he believed it was time for someone else on the left to step up. “I have done it twice already and had a heart attack a couple of years ago. We turned to Jeremy and said: ‘Come on, it is your turn, you have a go’,” he said.

Despite his crucial role in the winning campaign, McDonnell’s proposed appointment to the shadow cabinet has caused tension behind the scenes among MPs, with some holding back from confirming that they would serve under Corbyn if he was getting the job.

For a start, his manner is seen as less conciliatory than Corbyn. But there was also concern that some of his past statements and associations could prove controversial. In 2003, he sparked controversy with a call to honour the “bravery and sacrifice” of the IRA. At a leadership hustings in 2010, it was reported that he said he would be glad to go back to the 1980s to assassinate Margaret Thatcher.

Forthright and plain-speaking, he said in July that he would “swim through vomit” to oppose the Conservatives’ welfare bill. He is also such a fervent opponent of a third runway at Heathrow that he once picked up parliament’s ornamental mace in a breach of protocol and was temporarily suspended from the House of Commons.

In his new role as shadow chancellor, he will be charged with forging Corbyn’s anti-austerity message and key proposals such as quantitative easing to fund national infrastructure spending.

McDonnell is also a fierce critic of the response to the financial crash, supports renationalisation of the banks along with some other public services and described bankers as “jokers” during a campaign rally.