King Charles III's younger son and other high-profile figures allege that the publisher engaged in illegal activities, including phone hacking, at its titles and are seeking damages.
The case is one of several that Harry has brought against British newspaper groups since stepping down from royal duties in early 2020 and relocating to the United States.
The MGN trial, which is expected to last up to seven weeks, kicked off last month, days after Charles's May 6 coronation which Harry attended.
The California-based prince also made a surprise appearance at the High Court in March for a privacy claim he and others have launched against Associated Newspapers (ANL), publisher of the Daily Mail.
Harry, the Duke of Sussex, made written submissions in that case but did not give in-person evidence, sitting near the back of the court during several days of proceedings.
His appearance on the stand, expected Tuesday, is said to be the first time a senior royal has given evidence in court since Edward VII, who took the stand in an 1890 slander trial before becoming monarch.
Harry, who is fifth in line to the throne, has had a difficult relationship with the media, especially since he and his American wife Meghan left Britain.
As well as filing multiple lawsuits, the couple has repeatedly lashed out over alleged privacy invasions by photographers in particular.
Just weeks ago, they claimed to have been involved in a "near catastrophic car chase" with paparazzi in New York, an incident police and other officials played down.
Harry's mother, Princess Diana, was killed in a 1997 Paris car crash as she was being pursued by photographers.
He has also challenged the UK government in court over his security arrangements when he is back from the United States.
But on May 23, he lost his bid for a legal review of a decision refusing him permission to pay for specialist UK police protection himself.
In television interviews and his explosive memoir "Spare" -- released in January -- Harry hit out at other royals, accusing them of colluding with the press.
In court filings unveiled in April, Harry claimed the royal family as an institution had struck a "secret agreement" with one UK publisher that had prevented him from suing, to avoid a royal entering the witness box.
He also alleged the monarchy wanted to prevent the opening of a "Pandora's Box" of negative coverage that could tarnish the royal brand.
The MGN case centres on claims its tabloids conducted unlawful information-gathering, including voicemail tapping, to obtain stories about Harry and other high-profile figures.
The other claimants are two television soap opera actors and the ex-wife of a comedian.
At the outset of the trial on May 10, MGN apologized and admitted to "some evidence" of unlawful information-gathering and assured that "such conduct will never be repeated".
But it denied voicemail interception and argued that some claims had been brought too late.
The claimants' lawyer David Sherborne submitted that "industrial scale" illegal activities were happening at MGN and had been approved by senior executives.
Harry's unofficial biographer Omid Scobie -- who co-authored a best-selling 2020 book about Harry and Meghan -- claimed in a submission that he was shown how to hack voicemails while on work experience at MGN title The Sunday People.
Scobie also said that while on work experience at its sister paper The Mirror he overheard the then-editor Piers Morgan being told that information for a story about Australian pop star Kylie Minogue had come from voicemail.
Morgan, editor of the tabloid between 1995 and 2004, has denied any involvement in phone hacking.
© 2023 AFP