McTernan was born in London and grew up in Edinburgh, and attended Firrhill High School.

Accusations were made that Gillard had not attempted to find a suitable Australian candidate for the director of communications post, but had flouted Australia's visa process to employ John McTernan, a British citizen, on a 457 visa – a foreign workers scheme which is designed for employers who cannot find local candidates to fill jobs.

[16][17] In January 2015, McTernan was appointed chief of staff to Jim Murphy, then Scottish Labour leader, ahead of the 2015 general election, and oversaw media and policy in this role.

[21] Following the 2016 revelations about David Cameron's earlier offshore earnings, and Corbyn's call for an investigation,[22] McTernan argued in his Daily Telegraph column that tax avoidance is an expression of basic British freedoms.

[25] In November 2016, McTernan bet blogger Stuart Campbell $100 on a Clinton victory in the 2016 US presidential election, which he lost.

[26] In August 2017 McTernan joined the Labour Party left-wing pressure group Momentum, set up to support Jeremy Corbyn's leadership.

He went on to argue that Brexit was the result of a lack of a humanitarian intervention in the Syrian civil war because it would have prevented the refugee crisis.

[citation needed] In November 2024, Prime Minister Keir Starmer distanced himself from McTernan following comments he made on GB News while discussing inheritance tax rises in the budget which would affect farmers.

He said that that family farming is “an industry we can do without" and suggested that if farmers protested "we can do to them what Margaret Thatcher did to the miners”.