It is a truth almost universally acknowledged that what the average civilian really craves in life is a little proximity to celebrity. If a star does grant access, then a hitherto hidden portal abruptly opens up. Entering it can prove easier than exiting.

In 1971, Elliot Mintz was a twentysomething radio host in Los Angeles who one day interviewed the avant-garde artist Yoko Ono on his show. Ono was used to existing within the shadow of her more famous husband, John Lennon, and so was thrilled to claim a little solo limelight. She liked Mintz, and called him up the next day for a chat. This soon became a daily event, and she then recommended that John call him, too. Mintz was a good listener, patient, irrespective of his own schedule. If they rang in the middle of the night and woke him up, so what? They liked that he pandered to them, didn’t fawn. Then, when they’d run out of conversational steam, they’d simply hang up on him. “They rarely bothered to say goodbye,” he notes.

So began a relationship that would endure, with John up to his death in 1980 and with Yoko to this day. Previously, their acquaintance had remained that rarest thing in celebrity circles: private. But no more. Mintz, now 79, has decided to tell all. “I loved them like family,” he writes in this memoir. “I’d like to say they felt a similar familial attachment to me, but I was never completely sure.”

This didn’t trouble him too much, however. He installed another phone line in his house to ensure he’d never miss a call, and if it rang when he had a lady friend over, then she was asked quickly to leave. He was now otherwise engaged.

He couldn’t help himself; they fascinated him. “The English language contains some 170,000 words, and I’ve never come across a single one of them that fully describes the odd contours that made up my relationship with them,” he writes in a rapturous sigh. I can think of a word: gofer. It’s something he himself pretty much ’fesses up to later on: “I was a trusted confidant, a fixer, media rep, sounding board, and [their] connection to the outside world.”

He had come into their lives at a difficult time. Their marriage was in trouble. Both were coming down from heroin, then methadone. They were spiky, rude, self-involved. Sometimes, Lennon would summon him to keep him company, to drive him someplace, or to help him set up assignations with other women. Ono pleaded with him to keep her errant husband safe, while Lennon told Mintz: “There will be times when you’ll think she’s bloody mad. Just do what she tells you.”

He is there for the binges, the recrimination and the heartbreak. He is there, too, in the hours after Lennon’s murder, his presence requested by his widow to catalogue all his personal items.

For Beatles aficionados, there is little new here, but it does nevertheless offer up a fascinating ringside seat into the psychodrama of the most compelling celebrity couple of the 1970s, Lennon craving world peace and self-destruction, Ono communing with psychics and obsessed with the number nine. Throughout, Mintz displays a sense of duty that would impress Julian Fellowes for its Downton Abbey-esque obsequience. He could play Carson with aplomb.

The reader repeatedly wonders: why? Why such indulgence? He wasn’t even much of a Beatles fan: “I preferred Elvis.” Even Mintz isn’t entirely sure. “If only I had learned to say no, I might have [lived] a more balanced, traditional existence,” he writes. “I might have married, had children, made some ordinary friends.”

At one point he suggests that “in my more mystical moments, I ponder if we were bound by a ‘soul contract’ that made our friendship cosmically inevitable, forever to be repeated in various incarnations for all eternity.” Or perhaps he simply enjoyed being fame-adjacent, the chosen one?

The world does not need yet another take on either John or Yoko, but amid the countless critical tomes about them, this one is at least reverential and kind, albeit so blurred by the adoration of its subjects that objectivity is impossible.

Yoko, you feel, would approve.

• We All Shine On: John, Yoko & Me by Elliot Mintz is published by Bantam (£25). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply