He spent much of the following two decades - through the conspiracy theories, endless investigations and royal family turmoil - remaining mostly silent about her death.
He went to war in Afghanistan and watched his older brother marry.
Then in his late 20s, Harry crumbled, the prince told the Daily Telegraph in a recent, revealing interview that lent "unprecedented insight into his past."
At royal engagements, Harry found himself overcome by a "flight or fight" sensation. The prince felt angry, he told the Telegraph, as if he were "on the verge of punching someone." But he didn't understand what was causing the eruptions.
"I just couldn't put my finger on it," Harry told the Telegraph. "I just didn't know what was wrong with me."
It wasn't until he began speaking with friends and family, then a therapist, that Harry realized it was the unattended, unresolved grief of losing his mother so young that was possibly crippling him.
"I can safely say that losing my mum at the age of 12, and therefore shutting down all of my emotions for the last 20 years," Harry said, "has had a quite serious effect on not only my personal life but my work as well."
The interview, published April 16, was with the Telegraph's Bryony Gordon, who after her own struggles with obsessive-compulsive disorder began a podcast called "Mad World: Why it's totally normal to feel weird." The purpose of the podcast is to have unvarnished conversations about depression, anxiety and mental health.
Her first guest was Prince Harry.
In an essay she wrote about landing the interview, Gordon said she was surprised by how candid their conversation was in Kensington Palace.
"Was this really happening? I mean really really?" Gordon wrote in the essay. "Were we finally living in an era where even a representative of one of the most buttoned-up, traditional institutions on the planet might feel able to talk about the troubles in their head?"
It's not entirely surprising that Harry was willing to come on Gordon's show. He, alongside his brother and sister-in-law, the Duke and Duchess of Cambridge, founded a mental health charity called Heads Together in May 2016.
The three have actively worked to alleviate mental health stigma.
During a barbecue hosted by Harry, now 32, at Kensington Palace last year, he said: "I really regret not ever talking about it," referring to his mother's death. But Harry's conversations about mental health at the event took on an advocate's tone.
His interview with Gordon was more intimate.
The prince said he spent his teenage years and 20s dodging thoughts of Princess Diana, whose death came at the same time the royal family was preparing to introduce Prince Charles' and his longtime love (and current wife) Camilla as a couple.
"My way of dealing with it was sticking my head in the sand, refusing to ever think about my mum, because why would that help?" Harry told Gordon. "[I thought] it's only going to make you sad, it's not going to bring her back."
In the mid-2000s, Harry joined the British Army around the same time his boyish and drunken antics invited a ruthless media frenzy. He eventually rose to the rank of Apache helicopter commander and was praised for his skills by the British Defense Ministry.
But his service opened the door for speculation that his mental health challenges were related to his time overseas.
"I can safely say it's not Afghanistan-related," Harry said in the Telegraph interview. "I'm not one of those guys that has had to see my best mate blown up next to me and have to apply a tourniquet to both their legs. Luckily, thank God, I wasn't one of those people."
It was the way he had been forced to publicly grieve Princess Diana - and as a result hardly grieve at all - that did the most damage, the prince said. He didn't let his "emotions be part of anything, he said, and tried to behave like a "typical" 20-something.
"And then started to have a few conversations and actually all of a sudden, all of this grief that I have never processed started to come to the forefront and I was like, there is actually a lot of stuff here that I need to deal with," Harry told Gordon.
His brother, Prince William, who is expected to take over the throne one day, was a "huge support," Harry said, and encouraged him to talk with a professional.
As a way to mitigate the rage, Harry took up boxing, which he joked in the interview was better than hitting someone who didn't deserve it.
Seeking help, he said, is important for those struggling with mental health, "not just for you but everybody else around you as well because you become a problem.
I, through a lot of my 20s, was a problem and I didn't know how to deal with it."
Now, Harry said, he is in a "good place."
During her own essay about the interview, Gordon reflected that the prince's candid words were groundbreaking because " . . . in Britain, we don't talk about our feelings. We have bitten our lips, slapped on rictus grins, kept buggering on."
She continued: "It has always been a sign of strength and dignity to keep it all inside, and our Royal family have always been the embodiment of that, God bless them. But Prince Harry just redefined strength and dignity for a new generation."
And Gordon, she wrote, could "think of no more fitting tribute to his mother than that."
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)