Justice Tugendhat said that Chinese actress Tinglan Hong felt that she could not leave her home without being followed and hassled by photographers.
She was granted an injunction on November 11, which forbids any photographs to be taken outside of her home, or in the street.
The high court judge said that Hong was forced to change her phone number in recent weeks after she received threatening calls from journalists.
She also had 10 or more people camped outside of her house, sometimes waiting all night in the hope of securing a lucrative picture.
The identity of the defendants are unknown, but were simply referred to as the "person or persons responsible for taking photographs of the claimants outside their home and in the street during November 2011".
Justice Tugendhat said Hong feels that since the birth of her baby in September, "her life has become unbearable".
The judge added: "While Hugh Grant is very well known, the first claimant [Ms Hong] has never sought any publicity or been known to the public for any reason.
"She and Hugh Grant did their best to keep private the fact that the second claimant was their child and do not know how the information reached the public domain."
The said that the article was "illustrated by photographs which the first claimant recognised as having been taken as long ago as January 2011".
"At that time she had no idea that she was being followed and being photographed without her knowledge," the judge added.
Justice Tugendhat said that Hong had received a number of threatening phone calls after Grant appeared on BBC Question Time to discuss the phone hacking scandal at the News of the World.
He said she was "terrified" after a voice on the other end of the phone told her to "tell Hugh Grant to shut the f**k up". She had no idea how they had got her number, which she has since changed.
In a statement given to Hong's solicitor Mark Thomson, Grant said that he had confronted the photographers on November 3, asking if there "was anything he could do or say to make them leave a new and frightened young mother in peace".
They told him to show them the baby, to which he refused and asked why they thought it was acceptable for people to harass a new mother for commercial profit.
But, he said that the photographers just "shrugged and took more pictures".
Details of the press intrusion emerged this morning in a written judgement handed down by Justice Tugendhat after Grant successfully applied for an injunction under the Protection from Harassment Act against the photographers last Friday.
Thomson has also lodged a complaint on Hong's behalf with the Press Complaints Commission.
Grant is among a number of high-profile figures due to appear before Lord Leveson's inquiry into press standards following the phone hacking scandal at the News of the Word. The evidence of harassment of his former girlfriend was presented to the Leveson Inquiry on Wednesday.