Another hearing in her case against the Mail on Sunday publisher Associated Newspapers Ltd (ANL) has been fixed for March 2 "to decide matters consequential on this judgment, and directions for the next steps".
But the ruling means Meghan will not have to face her father in a high-profile court showdown, which she had been trying to avoid.
The 39-year-old sued ANL for privacy copyright and data protection over five articles published in February 2019 relating to the letter.
Her lawyers said the privacy claim should be resolved without a trial, arguing that the paper's publisher has "no prospect" of defending her claim for misuse of private information and alleged breach of copyright over the publication of the letter.
Meghan called for a "summary judgment", scrapping the need for potentially embarrassing trial requiring witnesses and a possible face-off with her estranged father, who had said he was willing to give evidence for the Mail on Sunday if the case went to trial.
ANL had said the facts of the case could only be determined by a trial in which Thomas Markle, Meghan and her former staff could have had to take the stand.
The High Court judge on Thursday afternoon UK time ruled in Meghan's favour.
The privacy row centred around extracts from the handwritten letter the duchess sent to her 76-year-old father in August 2018.
She is said to have felt forced to write the "painful" letter after they reached "breaking point".
Over two days in late January, the High Court heard submissions from Meghan's lawyers on their case for a summary judgment.
Justin Rushbrooke described the 1250-word letter as "a heartfelt plea from an anguished daughter to her father", during a two-day hearing.
But Markle claimed in a witness statement that the letter was a "criticism" of him.
He also referred to an article in People magazine, which mentioned the letter and featured five anonymous friends of Meghan.
One friend was quoted as saying: "After the wedding she wrote him a letter.
"She's like, 'Dad, I'm so heartbroken. I love you. I have one father. Please stop victimising me through the media so we can repair our relationship.'"
Markle branded this a "total lie" in his witness statement and said the article was "expressly authorised by Meg or she had at the very least known about and approved of its publication".
He said the quote "suggested to people that Meg had reached out to me with the letter, saying in the letter that she loved me and that she wanted to repair our relationship.
"That suggestion was false. The letter was not an attempt at a reconciliation. It was a criticism of me.
"The letter didn't say she loved me. It did not even ask how I was. It showed no concern about the fact I had suffered a heart attack and asked no questions about my health.
"It actually signalled the end of our relationship, not a reconciliation."
A judge at the High Court in London said the Duchess of Sussex "had a reasonable expectation" that the contents of the letter to her father would remain private.
Markle said the article in People magazine wrongly accused him of telling "mistruths" and "contained other inaccuracies about me".
He said: "It was wrong for People magazine to say I had lied about Meg shutting me out – she had shut me out, as the letter from her showed."
A bid to name the five friends was dismissed in August.
In delivering his ruling, Judge Warby said "the only tenable justification" for the Mail on Sunday's interference "was to correct some inaccuracies" about the letter contained in the People article.
But the judge said "the disclosures made were not a necessary or proportionate means of serving that purpose", adding: "For the most part they did not serve that purpose at all. Taken as a whole the disclosures were manifestly excessive and hence unlawful."
Lawyers for the duchess told the High Court the publication of the "intrinsically private, personal and sensitive" letter was a "triple-barrelled invasion of her privacy rights".
Rushbrooke said the letter was not "a vicious or unwarranted attack" on her dad but was "a message of peace".
The last line — included in court documents — read: "I ask for nothing other than peace and I wish the same for you."
The court papers add: "It is a heartfelt plea from an anguished daughter to her father [the word 'pain' or 'painful' appears no fewer than five times], begging him to stop talking to the press."
Rushbrooke said: "It was written, in short, by a daughter who felt she had reached a breaking point with her father."
The barrister told the court Meghan had sent it to Markle "at his home in Mexico via a trusted contact ... to reduce the risk of interception".
He described its subsequent publication as a "plain and serious invasion" of privacy.
Lawyers for ANL claimed it was written "to defend her against charges of being an uncaring or unloving daughter".
ANL lawyer Antony White said Meghan having a "fear" her letter might be intercepted showed she "must, at the very least, have appreciated that her father might choose to disclose it".
The court also heard that texts between Meghan and a royal aide that could reveal the truth about her "painful" letter to her dad have been deleted.
Jason Knauf, who at the time was communications secretary to both Prince Harry and Meghan and Prince William and Kate, is said to have helped the duchess work on the letter to Markle.
The duchess said Knauf "provided feedback" in the form of "general ideas".
Her lawyer Ian Mill told the High Court that any texts between Meghan and Knauf would have been deleted after 30 days.
This led Adrian Speck, for ANL, to claim the paper may have published sections of the letter Meghan did not have "full copyright over".
He added: "She may have a revised copyright which may be specific to a particular paragraph. [If] Mr Knauf sent another one back with further changes, you would need to know what has come in term of actual text from those two individuals."
White told the court a letter from lawyers representing aides known as the "Palace Four" said they would be able to "shed some light" on the drafting of Meghan's letter to her father, which forms part of the privacy case.
White claimed there was a "real prospect" the duchess "will fail to establish she was the sole author in the copyright sense".
The barrister added: "No truly private letter from daughter to father would require any input from the Kensington Palace communications team."
But Mill branded the suggestion a "farce" and said the final draft of the letter is the only one still to exist because it was made on Meghan's iPhone.
Mill argued that "she and she alone" created a draft of the letter to her father "which she then transcribed by hand" in regards to her copyright claim.
He said the letter was "an original literary work in which copyright subsists and is owned by the claimant" and asked the court to "grasp the nettle and decide the issue at this hearing".
In a witness statement previously read before the court, Markle said: "I am a realist and I could die tomorrow.
"The sooner this case takes place the better."
The blockbuster case — one of several brought recently by the Duke and Duchess of Sussex against media organisations — was filed by Meghan's lawyers in September 2019. It was due to be heard at the High Court in January but was adjourned for nine months for a "confidential" reason.
A string of pre-trial skirmishes have erupted in the build-up – including Meghan's claims ANL had an "agenda" of publishing intrusive or offensive stories about her being struck out as "irrelevant" last May.
Earlier this month, the Duke of Sussex won substantial damages from the Mail on Sunday publisher over claims he had not been in touch with the armed forces after leaving the UK.
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