Prince Harry took the stand in a London court Tuesday in his case against UK’s Daily Mirror — while across the pond in a Washington DC courtroom, the US government will be answering questions about why the royal’s visa application shouldn’t be made public.

The 38-year-old Duke of Sussex is testifying in the first of five lawsuits he has brought against UK tabloids accusing them of hacking his phone and invading his privacy.

Harry is the first member of the British Royal Family to testify in court in over 100 years.

Meanwhile in the US, conservative think tank Heritage Foundation is suing the Department of Homeland Security seeking to obtain the records from the prince’s visa application to see if he hid that he’d used drugs or if he was given preferential treatment in the process.

A hearing is set for Tuesday in that case.

On Monday, opening arguments began in London in the first tabloid case, in which Harry is suing Mirror Group Newspapers for allegedly hacking his phone and hiring private investigators to spy on him in 150 articles form 1996 through 2011.

The suit, however, centers around 33 of those articles — including a 1996 piece in the Mirror reporting he felt “badly” about the divorce of his parents, the late Princess Diana and the since-crowned King Charles III.

Judge Timothy Fancourt had directed Harry to be in court Monday in case opening statements wrapped early and there was time for his testimony to begin.

But the royal was not in court for the first day of trial with his lawyer citing Harry’s daughter Lilibet’s second birthday Sunday as the cause for his delay — which Fancourt said he was “a little surprised” by.

During opening remarks Monday, Harry’s lawyer David Sherbourne said the most minute and private details about his client’s life were made public by the Mirror Group Newspapers — including his injuries, love life and drug use.

“Nothing was sacrosanct or out of bounds and there was no protection from these unlawful information-gathering methods,” Shelbourne said.

But defense lawyer Anthony Green denied that Harry’s voicemails were listened in on by Mirror Group reporters and said it would have been nearly impossible to spy on him — given the robust security surrounding a royal member.

He maintained that the articles were based on legitimate reporting.

“There is simply no evidence capable of supporting the finding that the Duke of Sussex was hacked, let alone on a habitual basis,” said Green. “Zilch, zero, nil, nada, niente, nothing.”

Harry has accused the UK press of tormenting him — blaming the paparazzi for causing the 1997 car crash that killed his mother and saying the relentless media scrutiny drove him and wife Meghan Markle to cast off the royal life and move to the US in 2020.

The prince has laid out his gripes in his memoir “Spare” and in interviews with Oprah Winfrey and others.

In the tell-all book, Harry admitted to using drugs including cocaine, marijuana and psychedelic mushrooms — prompting the US court case over his visa papers since drug abusers are “inadmissible” for visas.

Officials can make exceptions to that rule.

Heritage Foundation filed a Freedom of Information Act lawsuit to see if the DHS properly approved Harry’s application and to see whether the decision should be reviewed in light of the admissions he made in his book.

In the London case, Green has said he plans to question the royal for a day and a half.

Harry suffered “huge bouts of depression and paranoia” caused by his fears that friends and acquaintances betrayed him by leaking information about him to the press, he said in court papers.

But he later came to realize the stories were obtained through aggressive and illegal reporting and resulted in romantic and family relationships falling apart, he claimed.

Mirror Group has admitted and apologized for one incident in which it hired a private investigator to look into one of Harry’s nights out at a bar that culminated in a 2004 pieced headlined, “Sex on the beach with Harry.”

With Post wires