“Give me a break!” were the first words Lieutenant William Calley Jr, accused of killing 109 Vietnamese civilians, said to Seymour Hersh when the intrepid reporter finally found him. It had not been an easy task for Hersh, who had been chasing the story for weeks. In the hunt for Calley he had driven to Fort Benning, Georgia, scoured (without success) endless volumes of phone directories, broken into a military barracks and pretended to be a lawyer. When he finally did find Calley, the last thing he was willing to give him was a break. That initial story of what had happened at My Lai, and how complicit the US army had been in the killing of civilians, came after an all-night, bourbon-fuelled interview, at the end of a months-long quest.

William Calley Jr at a pre-trial hearing prior to his court martial for his involvement in the My Lai massacre. Photograph: Bettmann Archive

But Hersh was not about to catch a break either. Instead, there was a surly lesson: the truth, however doggedly procured, does not guarantee publication. His story, which would eventually win him the Pulitzer prize in 1970, was rejected not by one or two but three magazine editors. Hersh, understandably, was “devastated by the amount of self-censorship” he was encountering in his profession. Indeed, he would find a lawyer before he would find a publisher, and the latter was Hersh’s 23-year-old next-door neighbour. Yet, for all its birthing pains, the story of the My Lai massacre put Hersh among the big boys (and they were at the time mostly boys; the first female name, that of New York Times journalist Gloria Emerson, doesn’t appear in this book until page 161). Not just the Pulitzer but other awards came his way, but a job was less forthcoming, and it took the publication of his second book on the massacre to get that. His exposé, which was verified by subsequent accounts, had by then been verified by many others who came forward. It had not stopped the war, but it certainly forced a reckoning.

Later Hersh got more prestigious reporting gigs, at the New Yorker first and then the New York Times. His reputation as a rebel went with him. In one telling and startling exchange, Abe Rosenthal of the New York Times comes up behind him in the newsroom, ruffles his hair and says: “How’s my little commie?” It is a supposedly gentle reminder that Hersh was not to let his feelings about the war get in the way of his reporting of it. The truth, as he notes in Reporter, that his feelings about the war didn’t begin with ideology, seemed not to matter.

It is perhaps inevitable that the reminiscences of a journalist who has been so long at the craft promote the idea that nothing, in journalism or elsewhere, ever quite changes. So it was with one of Hersh’s biggest post-My Lai stories. In 1974, after some nocturnal squabbling with Rosenthal over length, the New York Times published Hersh’s exposé of CIA domestic spying. Instrumental in the programme was James Jesus Angleton, “a fabled character inside the CIA”, known “for his belief that the Russians had completely penetrated the Agency and for his willingness to investigate anyone”. Few were pleased with Hersh’s revelations; the incensed right denounced him as a “Red”.

Seymour Hersh in New York, 1997. Photograph: Michael Schmelling/AP

It wasn’t until 11 September 2001 that Hersh underwent, in the words of the same paper, an “evolution” (the article also pointed out that some of the “most controversial and startling information” that appeared in the tumultuous months after 9/11 had done so under his byline). Yes, his “critical books on Henry A Kissinger, the Central Intelligence Agency and John F Kennedy” had made him “a permanent outsider”. But his attitude to the CIA had “shifted. In 1974, he exposed it as an out-of-control, rogue agency illegally spying on Americans. Now, through his eyes, it seems emasculated, Prometheus bound by bureaucrats”. Hersh had been arguing in the New Yorker that the CIA was just “not up to the job”.

His “major” stories from the war on terror era show an even balance between those that focus on not trusting other countries (Pakistan, Saudi Arabia and Israel) and ones that consider American overreach (Dick Cheney’s obsession with bombing Iran, the CIA’s obsession with supporting Israel against Hamas). But whether Hersh intends it or not, by the end of Reporter there is a sense of having come full circle. One of the last stories, whose investigation he describes here, is of the 2011 killing of Osama bin Laden. According to Hersh’s sources in Pakistan, the Obama administration, eager for a rousing military victory to carry them through the 2012 election, worked with the Pakistanis who had, in Hersh’s telling, long kept Bin Laden captive. Then, at the last moment, the administration carried out the raid themselves, betraying the very people that had helped them set it up.

David Remnick, then Hersh’s editor at the New Yorker, refused to publish, pointing to a lack of reliable information. Hersh later learned that the magazine had committed to another story written from the perspective of one of the Navy Seals who took part in the raid. (His piece was eventually published by the London Review of Books.)

So the point Hersh makes about journalism and self-censorship early in these pages echoes again in the last. His anger now cooled, he sums it up: “editors get tired of difficult stories and difficult reporters”. What he, in his moment of equanimity leaves out, is that it is just those stories written by just those reporters that readers want, and of which they never tire.

• Reporter: A Memoir is published by Allen Lane. To order a copy for £17 (RRP £20) go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.