Baggio Leung and Yau Wai-ching deliberately misread their oaths of office, inserted expletives and draped themselves with "Hong Kong is not China" flags during a swearing-in ceremony last month, prompting a judicial review into their future as legislators.
"Mr. Leung and Ms Yau have been disqualified from assuming and have vacated the office of a member of the Legislative Council," judge Thomas Au said in a written judgement.
"The oaths purportedly taken by Mr. Leung and Ms Yau on October 12 2016... are invalid and void and have no legal effect," Au said.
Au also said the way they took their oaths showed "clearly that they did not truthfully and faithfully intend to commit themselves to uphold and abide by" the city's constitution.
"With or without (Beijing's) interpretation, the court would reach the same conclusion," he added.
Yau told a media scrum outside the courthouse that the ruling did not come as a surprise to her.
"I knew that there was this possibility... the government has used so many small acts to suppress the courts and the courts had so much pressure and came up with such a decision," she said.
The High Court's decision was preempted by Beijing's ruling last week, which said that any oath taker who does not follow the prescribed wording of the oath, "or takes the oath in a manner which is not sincere or not solemn", should be disqualified.
That move was slammed by pro-democracy activists and legal experts as a massive blow to Hong Kong's judicial independence and sparked demonstrations by both pro-Beijing and pro-independence groups.
Ahead of the court ruling, the city's leader called for zero-tolerance against activists pushing for independence from China.
"Those who are advocating for independence and other forms of splitting from the country are a small minority but I cannot lower my guard and cannot (give them) any tolerance," Leung Chung-ying told Xinhua Monday.
"Members of the Hong Kong independence (movement) cannot appear in the political system," Leung said.
Hong Kong was handed back to China by Britain in 1997 under a "one country, two systems" deal which protects its freedoms for 50 years, but there are growing concerns those liberties are disappearing.
