Born in Rotherhithe, then an impoverished area of London with appalling housing conditions, Haines was the youngest child of a dock worker who died when he was 2. His mother, a cleaner at a hospital, brought up the family. He joined the Labour Party as a teenager. At 14, Haines became a copyboy on the Glasgow Bulletin, and then a lobby reporter at Westminster in 1950.
In 1954 Haines became the political correspondent for George Outram & Co. in Glasgow, before moving to Edinburgh around 1960 to work for the Scottish Daily Mail. From 1964 he was employed by the pre-Murdoch Sun, and became Harold Wilson's press secretary in 1969.
In 1974 Wilson had a health scare over a racing heart complaint, but "I told the press, who believed me when I said that Harold had the flu," Haines recalled in 2004. "We had an economic crisis and we had a majority of three", he explained.
In Glimmers of Twilight (2003), Haines claims that Wilson's doctor Joseph Stone offered to murder Marcia Falkender, the head of Wilson's political office, after she attempted to blackmail Wilson over an affair they had twenty years earlier. The BBC, in an out-of-court settlement with Falkender, paid her £75,000 after these claims were repeated in The Lavender List, a drama documentary written by Francis Wheen and broadcast in 2006. Although Haines himself has not been sued, as a libel action involving him as the source it is generally accepted that the BBC settled because the original claimant would not stand behind the story. Roy Hattersley later referred to Glimmers of Twilight as a "book of tall tales". The allegations relating to Stone were repeated in the BBC's documentary The Secret World of Whitehall (2011).
Not long after Wilson's resignation as Prime Minister, Haines published a book The Politics of Power about his experience of British political life. Attention mainly concentrated on two chapters about Marcia Williams (now Falkender) and her influence. Haines claimed that Ms Williams' troublesome presence had been the real cause of Wilson's resignation. What he wrote in the book denied Wilson's statement at the time of his resignation that when he came back to power in 1974, he had told the Queen that he would not continue after he had reached the age of 60. Some commentators (e.g. Brian Sedgemore) considered that The Politics of Power was an interesting account, but the chapters about Marcia Williams were the weakest in the book.
In a 2010 interview, Haines claimed that in the aftermath of the February 1974 general election, Harold Wilson had planned to discredit Liberal leader Jeremy Thorpe by exposing Thorpe's relationship with Norman Scott in the event of the Conservative government reaching an agreement with the Liberals that would have permitted it to remain in power.
Haines turned down a peerage from Wilson in the 1976 Prime Minister's Resignation Honours in part, he claimed, because he did not wish to be awarded one in a list also consisting of Joe Kagan and Eric Miller, already under suspicion of criminal activity at the time.
In 1976 he joined the Daily Mirror. At the time Robert Maxwell purchased Mirror Group Newspapers on 12 July 1984 he told a meeting of his colleagues that their new boss "is a crook and a liar – and I can prove it".
Appointed the Mirror Group's political editor shortly after Maxwell's purchase of the Group, he also became a non-executive director of the board, and from 1984 to 1990 he was the Mirror's assistant editor. In 1988 the authorised biography by Haines of Robert Maxwell was published. The Mirror's then owner had commissioned the work to pre-empt a biography by investigative journalist Tom Bower, which Maxwell unsuccessfully attempted to have withdrawn. Haines' biography was generally considered to be encomium and was treated with a mixture of ridicule and extreme criticism by the media at the time of its release – The Times referred to it as "notorious". According to Tom Bower, Haines' biography was so flattering Maxwell would give out copies instead of business cards. A report in 2001 by the Trade and Industry Department inspectors into the collapse of Maxwell's business empire found that Haines "had accepted the position [with Maxwell] and ought to have discharged the responsibilities that went with the position. He therefore bears a limited measure of responsibility" for the debacle.