Katie Rowley, 36, from Leeds, started court action after the sessions went ahead without interpreters on screen.
The government denied breaching its legal obligation to make broadcasts accessible to deaf people.
In his ruling, a judge said the lack of provision constituted discrimination.
Ms Rowley launched the court action against Cabinet Office Minister Michael Gove in relation to the "data briefings" on 21 September and 12 October 2020.
She had argued that being unable to access the official information had caused her stress and affected her wellbeing.
Finding in her favour, Mr Justice Fordham said: "The lack of provision - the provision of subtitles only - was a failure of inclusion, suggestive of not being thought about, which served to disempower, to frustrate and to marginalise."
Though he agreed with Ms Rowley's claim in respect of both, the judge said subsequent briefings were not in breach of equality legislation.
The level of damages awarded to the claimant would be assessed by a county court judge, he added.
After the ruling, Ms Rowley thanked the judge and her legal team and said she was "very happy with the outcome".
She said she was "emotional that we have achieved what we needed to be equal but sad at the same time that we had to fight for our rights".
Similar briefings in Wales, Scotland and Northern Ireland included British Sign Language interpreters on screen.
Ms Rowley's solicitor, Chris Fry, of Fry Law, said: "This significant judgment ensures the Cabinet Office must ask itself, 'Where is the interpreter?' as part of its planning for its broadcasts.
"It is remarkable that while finishing writing its National Disability Strategy document announced today, it was instructing its own lawyers to argue that subtitles were a suitable alternative - a proposition criticised as showing prejudice," he said.
Mr Fry said there were another 260 pending cases against the Cabinet Office in relation to the lack of any interpreters for the beginning of the pandemic.
A Cabinet Office spokesman said after the ruling: "We are pleased that the court ruled our policy of using on-screen British Sign Language interpreters was lawful during the pandemic.
"Our priority has always been to reach the largest possible audience with important public information, and we will continue to ensure that British sign language interpretation is made available during Covid-19 briefings."
Officials said there had been more than 170 Covid briefings and "only two" had been found to be unlawful because British sign language was not provided on screen.
Ms Rowley, who was 25 weeks pregnant when she launched the judicial review claim, previously said the stress caused by being unable to access information at the briefing impacted upon her wellbeing.
Many who use BSL as their first language say they cannot rely on subtitles because the average reading age for deaf people is nine years.
"I have dyslexia myself - I am a slow reader - so that means when I was reading the subtitles, I would miss so much information and [it] would just mess up my head. It would be so difficult - it became impossible," Ms Rowley previously told the BBC through an interpreter.