Prince Harry is among high-profile figures accusing the publisher of using private investigators and phone hacking to gain access to stories about them.
His barrister David Sherborne said millions of pounds were paid to private investigators, with the payments signed off by senior figures at MGN.
MGN denies the claims.
It is alleged that journalists from the Daily Mirror, Sunday Mirror and Sunday People newspapers obtained private and confidential information about people's lives through a variety of illegal means.
The bulk of the trial's evidence are 207 newspaper stories, published between 1991 and 2011 - some 67% of which were written about Harry, the Duke of Sussex.
Mr Sherborne told the High Court one of the most "serious and troubling" features of the case is the extent to which "widespread, habitual and unlawful" activities were "authorised at the highest level".
This included "the systemic and widespread use of PIs (private investigators) by MGN journalists to unlawfully obtain private information" of various individuals, Mr Sherborne told London's High Court.
Mr Sherborne has referred the court to key senior figures in MGN who he claims "authorised" the unlawful obtaining of information.
He said this included former editors Piers Morgan, Neil Wallis, Tina Weaver, Mark Thomas, Richard Wallace and Bridget Rowe, and alleged that managing editors and senior executives also knew.
"Mr Morgan was right at the heart of this in many ways," Mr Sherborne told the court. "He was a hands-on editor and was close to the board. We have the direct involvement of Mr Morgan in a number of these incidents."
Mr Morgan was Daily Mirror editor from 1995 until 2004.
Mr Sherborne said the alleged unlawful activities also included MGN journalists intercepting landline voicemails, even if the phone numbers were ex-directory - meaning they were not listed in the telephone directory and the phone company would not provide them to those who asked for them.
Claims brought by Harry and three others are being heard in the trial, expected to last six to seven weeks, as being "representative" cases of the types of allegations facing the publisher.
The other claimants are former Coronation Street actors Nikki Sanderson and Michael Turner, known by his stage name Michael Le Vell, and comedian Paul Whitehouse's ex-wife Fiona Wightman.
They are all expected to give evidence - when the prince does so in June, he will become the first senior member of the Royal Family to appear in court and be cross-examined in modern times.
The four cases were chosen by the trial judge to help the court set the level of damages MGN should pay if the claimants win, as well as establish the various allegations facing the publisher.
The court would then consider other cases from celebrities including the former Girls Aloud singer Cheryl, actor Ricky Tomlinson, former Arsenal and England footballer Ian Wright and the estate of late singer-songwriter George Michael.
MGN has denied the allegations, including those of voicemail interception.
In its defence against some of the claims made by Prince Harry, MGN's lawyers argued that he did not have "a reasonable expectation of privacy".
This argument was made in response to articles about his relationship with Chelsy Davy - the break-up of which Harry blamed on press intrusion, his alleged drug use and one that reported he was forced to carry out farm work as punishment for wearing a Nazi uniform to a party.
In other instances it claimed published information was "limited and banal".
In response to one of the 33 articles put forward by Prince Harry's legal team, which gave details about his 18th birthday celebrations, MGN lawyers argued that the information came from an interview the duke gave to the Press Association.
The article published under the headline "No Eton trifles for Harry, 18" in September 2002 "simply repeated the details that the claimant [Harry] had given" including that he would not be having a party and would be spending the day with his father and brother, MGN argues in court documents. It said there was "no evidence of voicemail interception".
However on Wednesday, the publisher acknowledged and "unreservedly" apologised for a separate instance of unlawful information gathering against Harry, adding that the legal challenge brought by the prince "warrants compensation".
On Thursday, reporters saw the list of 33 stories at the heart of Prince Harry's claim for damages against MGN. He is relying on them to prove phone hacking and other unlawful activity against him. Here are some of them:
* The earliest is: "DIANA SO SAD ON HARRY'S BIG DAY", published in the Daily Mirror on 16 September 1996. MGN said this story, concerning the late Princess of Wales's divorce from King Charles III, was not private information, but that she had talked about her children on BBC Panorama
* "SNAP... HARRY BREAKS THUMB LIKE WILLIAM; EXCLUSIVE". MGN said the story from November 2000 had been confirmed by the Palace the day before
* "RUGGER OFF HARRY", reported the Sunday Mirror in November 2001 about an injury the young prince suffered on the playing fields of Eton. MGN said its story came from a "confidential Eton source" and there was no evidence of voicemail interception
* "HARRY TOOK DRUGS" and "COOL IT HARRY" appeared in the Sunday Mirror in January 2002. MGN denied phone hacking, and said the story first ran in the News of the World. It also said a member of the Royal Family had no reasonable expectation of privacy if they were taking drugs
In Thursday's hearing, Mr Sherborne discussed a Daily Mirror front page story from 1999, which revealed confidential details about the finances of Prince Michael of Kent - cousin of the late Queen Elizabeth II - including that he was in debt to a bank.
Prince Michael's lawyers later told MGN they had deduced that a "blagger" had called the bank and, posing as the royal's accountant, obtained confidential information.
MGN eventually settled the claim, published an apology and paid his legal costs, the barrister said.
"It's inconceivable, given the way this progressed, that the legal department and Mr Morgan were not well aware of the source of the story, and that it came from illegally obtained information," Mr Sherborne told the court.
Mr Morgan has consistently denied any knowledge of phone hacking during his time editing the newspaper, but this will be the first time a court has been asked to rule on claims about what he knew.
Speaking to the BBC's Amol Rajan before the trial began, Mr Morgan said he could only talk to what he knew about his own involvement, adding: "I've never hacked a phone, I wouldn't even know how."
Mr Morgan also pointed out he only worked for the Daily Mirror and had no responsibility for the Sunday Mirror, Sunday People or other titles.
In 2015, MGN admitted journalists had regularly used unlawful techniques to obtain private information - and issued a public apology.
The High Court ordered the publisher to pay out damages totalling £1.25m to eight phone-hacking victims, including more than £260,000 to the actor Sadie Frost.
The trial continues.