The Duke and Duchess of Sussex called her father and sent text messages in the days before their wedding, warning him that contacting the press would "backfire" and offering to help him, court documents reveal.
The duchess is bringing a privacy claim against Associated Newspapers, publisher of The Mail On Sunday and MailOnline, over publication of a "private" letter sent to Thomas Markle, 75, in August 2018.
Ahead of the first hearing in the case on Friday, Meghan's lawyers filed a reply to the publisher's defence, which sets out communications sent by the couple to Mr Markle shortly before the royal nuptials on May 19 2018.
The documents disclose texts sent on May 14 2018 after calls to Mr Markle went unanswered, in which Harry wrote: "Tom, Harry again! Really need to speak to u.
"U do not need to apologize (sic), we understand the circumstances but 'going public' will only make the situation worse.
"If u love Meg and want to make it right please call me as there are two other options which don't involve u having to speak to the media, who incidentally created this whole situation.
"So please call me so I can explain. Meg and I are not angry, we just need to speak to u. Thanks.
"Oh any speaking to the press WILL backfire, trust me Tom. Only we can help u, as we have been trying from day 1."
The documents state that, rather than responding to the calls and messages, Mr Markle issued a public statement through US website TMZ that he had gone to hospital because he had suffered a heart attack.
They also reveal this is how Meghan first learned her father was in hospital.
In a text to her father on May 15 2018, Meghan wrote: "I've been reaching out to you all weekend but you're not taking any of our calls or replying to any texts.
"Very concerned about your health and safety and have taken every measure to protect you but not sure what more we can do if you don't respond."
The duchess sent another text about 10 minutes later, offering to send a security team to him, adding: "Please, please call as soon as you can.. all of this is incredibly concerning but your health is most important."
Mr Markle responded by saying he would be in hospital for a few days but was OK, and refused the offer of security.
Harry then sent a further message from his wife's phone to provide Mr Markle with details of the security team, and asked his father-in-law to speak to him about letting a security guard return to Mr Markle's house, according to the court papers.
Barrister David Sherborne, who is representing Meghan in her claim against the publisher, said in the documents: "The claimant's husband pleaded with Mr Markle to let them help him."
The document claims the duchess was "unsure" if an "unpleasant message" was sent by her father or someone pretending to be him and accepts that, after further calls to him went unanswered, she had no further contact with Mr Markle.
It also alleges that statements made in the publisher's defence are "manifestly absurd" and "demonstrably unsustainable", and accuses Associated Newspapers of "harassing and humiliating" Mr Markle.
Mr Markle became embroiled in controversy after allegedly staging paparazzi photographs of himself in the run-up to the royal wedding.
At the time it was also reported Meghan had called her father and later texted him saying she loved him and was concerned about his health.
Meghan is suing the publisher over an article which reproduced parts of the handwritten letter sent to her father, the headline of which read: "Revealed: The letter showing true tragedy of Meghan's rift with a father she says has 'broken her heart into a million pieces'."
Sections of the letter were published in the newspaper and online in February last year, and it was announced the duchess would be bringing legal action in October.
Meghan's lawyers say the letter was "obviously private correspondence" which detailed "her intimate thoughts and feelings about her father's health and her relationship with him at that time".
They also allege that the newspaper "chose to deliberately omit or suppress" parts of the letter, which "intentionally distorted or manipulated" its meaning, and gave her no warning it was due to be published.
The duchess is seeking damages for alleged misuse of private information, copyright infringement and breach of the Data Protection Act.
Associated Newspapers wholly denies the allegations – particularly the claim that the letter was edited in any way that changed its meaning – and says it will hotly contest the case.
In its defence, lawyers for the publisher said: "The contents of the letter were not private or confidential, self-evidently or at all."
A preliminary hearing on Friday, which will be conducted remotely, will deal with an application by Associated Newspapers to strike out parts of Meghan's case ahead of a full trial of the issues.
Mr Markle has claimed he felt pressured to share the letter after its contents were misrepresented in a magazine article.
In an interview with the Mail On Sunday, he said: "I have to defend myself. I only released parts of the letter because other parts were so painful. The letter didn't seem loving to me. I found it hurtful."