In his book One Two Three Four, Craig Brown detailed the ways that the Beatles fleetingly touched and altered millions of lives. In that account, one figure occasionally steps out of the shadows: Mal Evans, bouncer at the Cavern Club, driver and bodyguard as the band travelled first down the newly tarmac-ed M6 and then way beyond. Evans was both a trusted insider to those helter-skelter years and, in some ways, the ultimate Beatles groupie. In this exhaustively detailed account of his truncated life – Evans died aged 40 in 1976 in a volley of gunfire from the LAPD after he had apparently waved a Winchester rifle in their direction seeking his own destruction – he finally assumes the place to which all walk-on actors privately aspire: centre stage.

The book has been a long time coming. At the time of his death, Evans had a publishing contract to tell the story of his life, based on diaries and notes of his time on the road. He left a draft of his memoir but that was blocked from publication by his widow, Lily. The archive and the manuscript were saved from the garbage by an alert temp at the publisher, who with the help of Yoko Ono saw that the material was returned to the family. Finally in 2020, Evans’s son, Gary, who had been 14 when his father died, commissioned the musicologist and Beatles obsessive Ken Womack to tell the full story.

It is a tale we know better, as John Lennon might have said, than the gospels – the early innocent days in Liverpool, the ear-splitting screams of Beatlemania, Ed Sullivan and the conquering of the US, the maharishi, the preternatural creativity and the eventual rifts and breakup – but here told through the eyes of an unheard disciple.

Evans recognised his calling immediately. There was a foundation myth to prove it. In January 1963, with the group’s second single Please Please Me rising in the charts, Evans stepped in to drive them to London for a TV appearance, when the regular roadie, Neil Aspinall, went down with flu. On the return journey, in a blizzard, the windscreen of their Ford bus shattered; Evans prevented a crash by punching out the glass and drove the long and winding road home with a paper bag on his head (with eyeholes) to prevent frostbite, while the Fab Four took it in turns to lie on top of one another in the back to keep warm. They never forgot that night, and from then on Mal – variously christened Malfunctioning, or Malcontent, or Malodorous – was a fixture in any and all magical mystery tours.

He was built for the task. A burly 6ft 3in, he had been nicknamed “hippo” at school in Liverpool, a moniker he warmed to “because it always seemed to be a fairly amiable, vegetarian type of animal”. He was the perfect Beatles muscle: looking the part in a scrum of fans, but always temperamentally prepared to give peace a chance. He was 27 when he fell into the group’s orbit, working as an engineer for the GPO in his day job, and with a baby son at home. To begin with, he was the big bloke at the Cavern on his lunch break who would request Elvis songs.

The generosity of spirit that characterised the Beatles on the road meant that Evans was always far more than a hired hand. He not only carried a doctor’s bag full of vital supplies – aspirin, condoms, guitar picks – and acted as a trusted conduit for drugs and women along hotel corridors, he was a confidant and participant in many of the group’s early adventures. When Bob Dylan famously turned the four on to marijuana at the Delmonico hotel in 1964, Evans was not only passed a joint but instructed by Paul McCartney to record for posterity the band’s first impressions of being stoned. He dutifully followed McCartney around with a notebook and pencil. The following morning they discovered he had written down a single sentence: “There are seven levels.” At the ashram in Rishikesh, he kept the press at bay and organised Ringo’s breakfasts and George Harrison’s birthday party, but also found time to compose a song with Donovan and go on a tiger hunt. He claimed that on that trip after many hours of uninterrupted meditation, McCartney informed him that he had experienced a vision of the roadie standing at his bedside, gently repeating words of comfort: “Let it be, let it be.”

As this 500-plus page history progresses, very slowly, toward its tragic ending – Evans and his family were inevitably undone by the intensity of these wild and whirling experiences – it becomes a kind of cautionary tale. Perhaps because the Beatles had made what they did look so easy, part of Evans came to believe he could do it, too. After the band’s breakup, he tried his hand as a producer and a songwriter (we have his own accounts of sessions with George and Ringo). By 1975, however, he was thrilled to be invited to be the guest star at “Beatlefest”, a fan event in New York. He was due to reminisce on stage for half an hour, but the crowd wouldn’t let him leave: “They had to drag me off in the end, because the audience was so ‘pro-me’,” he recalled. In many ways, this book recreates the shift of focus that event implied, the proximity Evans had to world-changing genius, and his distance from it. “It was like being a Beatle for a weekend,” he wrote at the time. It was and it wasn’t.

• Living the Beatles Legend: On the Road With the Fab Four: The Mal Evans Story by Kenneth Womack is published by Mudlark (£25). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply