He believes journalists from the Daily Mirror, Sunday Mirror and The People exploited a security gap to access his voicemails and hear messages left by friends and family.
As he entered the witness box, Harry's 55-page witness statement was published, detailing the times when he says journalists at the newspaper publisher used unlawful methods to gather information to generate stories about him, including phone hacking. MGN denies phone hacking in this case.
Here are some key extracts from his statement which he is being challenged on in court by barristers on Tuesday and Wednesday.
"In my experience as a member of the Royal Family, each of us gets cast into a specific role by the tabloid press.
"You start off as a blank canvas while they work out what kind of person you are and what kind of problems and temptations you might have.
"They then start to edge you towards playing the role or roles that suit them best and which sells as many newspapers as possible, especially if you are the 'spare' to the 'heir'.
"You're then either the 'playboy prince', the 'failure', the 'dropout' or, in my case, the 'thicko', the 'cheat', the 'underage drinker', the 'irresponsible drug taker', the list goes on.
"As a teenager and in my early 20s, I ended up feeling as though I was playing up to a lot of the headlines and stereotypes mainly because I thought that, if they are printing this rubbish about me and people were believing it, I may as well 'do the crime'.
"It was a downward spiral, whereby the tabloids would constantly try and coax me into doing something stupid that would make a good story and sell lots of newspapers.
"Looking back, such behaviour on their part is utterly vile."
Prince Harry says journalists would blag information about his former girlfriend Chelsy Davy's flights to the UK to see him. The couple were in an on-off, sometimes long-distance, relationship for six years from 2004.
"I walked into the [airport] arrivals hall with a baseball cap on and immediately spotted five separate paparazzi sitting on benches with cameras in bags, their hands inside rucksacks and everyone else looking at me," says the prince.
"I remember that someone was videoing me with one of those tiny little cameras between their legs.
"I recall thinking how on earth did they know I was going to be there, but now it's obvious.
"Here were five big, burly and dodgy looking men, with their hands in their pockets or in rucksacks and satchels in a busy public place.
"My security and I simply couldn't know whether they were reaching for a camera or drawing some kind of weapon."
Chelsy Davy and Prince Harry were in an on-off relationship between 2004 and 2010
He adds: "I always felt the tabloids wanted me to be single, as I was much more interesting to them and sold more newspapers.
"Whenever I got into a relationship, they were very keen to report the details but would then, very quickly, seek to try and break it up by putting as much strain on it and creating as much distrust as humanly possible.
"The twisted objective is still pursued to this day even though I'm now married."
"Tabloids would routinely publish articles about me that were often wrong but interspersed with snippets of truth.
"This created an alternative and distorted version of me to the general public - the people I had to serve and interact with as a member of the Royal Family - to the point where any one of the thousands of people that I met or was introduced to on any given day, could easily have gone: 'You know what, you're an idiot. I've read all the stories about you and now I'm going to stab you."
Prince Harry on a walkabout in Edinburgh with his wife, Meghan Markle, in 2018
Prince Harry says royal aides gave him his first phone when he went to Eton College, a boarding school in Windsor - and it became crucial to his daily life.
"As I was very heavily involved with various commitments, I would constantly be leaving and receiving voicemails, as text messaging was much less common back then," he says.
"It was my main means of communicating with my family [including my mother who I was obviously extremely close] ... my girlfriend at the time, my friends, members of the Royal Household and those I was working with.
"My voicemails would include incredibly private and sensitive information about my relationships, my operational security and that of my family [and in later years] my work both in the Army and as a senior member of the Royal Family."
He says knowing MGN journalists were listening in to private and sensitive voicemails suggests they could have heard "anything and everything".
This created huge stress, presented security concerns and created a "huge amount of paranoia" and suspicion in his relationships, he says.
"I felt I couldn't trust anybody, which was an awful feeling for me, especially at such a young age."
The King accompanied Prince Harry on his first day at Eton College in Berkshire
Prince Harry says numerous papers had reported a rumour that his biological father was James Hewitt - a man his mother had a relationship with after he was born.
At the time, he says, he wasn't aware of the timeline. Aged 18 and having lost his mother six years earlier, he says such stories felt were "hurtful, mean and cruel".
"I was always left questioning the motives. Were the newspapers keen to put doubt into the minds of the public so I might be ousted from the Royal Family?"
James Hewitt, a former cavalry officer, had a five-year affair with Princess Diana
A 2003 article by The People detailed a disagreement between Prince Harry and his brother, the Prince of Wales, over a potential meeting with their mother's former butler, Paul Burrell. He says the pair had strong feelings about Mr Burrell's indiscretion after he sold their mother's possessions and conducted interviews about her.
"We firmly believed that she would have expected some privacy in death, especially from someone she had trusted," he says.
William had wanted to set up a meeting with him - Harry was firmly against it, having made up his mind about Mr Burrell.
The article said he believed him to be a "two-faced shit", a phrase he believes could have been lifted from a voicemail message.
Paul Burrell said Princess Diana called him her rock
Breaking with the convention that royals never interfere with politics, Prince Harry attacks Rishi Sunak's government in his statement.
"Our country is judged globally by the state of our press and our government - both of which I believe are at rock bottom.
"Democracy fails when your press fails to scrutinise and hold the government accountable, and instead choose to get into bed with them so they can ensure the status quo."
Prince Harry says he is determined to see this action through to the end because he is convinced unlawful information gathering was known about by those at the top.
"The fact that it was not just the journalists who were carrying out the unlawful activity, but also those in power who were turning a blind eye to it so as to ensure that it would continue unabated - and who then tried to cover it up when the game was up - is appalling.
"The fact they're all ganging up to protect each other is the most disturbing part of all, especially as they're the mothership of online trolling.
"Trolls react and mobilise to stories they create. People have died as a result and people will continue to kill themselves by suicide when they can't see any other way out.
"How much more blood will stain their typing fingers before someone can put a stop to this madness."
-- Courtesy of BBC News