Mr Lai was one of several activists in court on Friday who were earlier found guilty of charges relating to large pro-democracy demonstrations in 2019.
The 73-year-old founder of Apple Daily is a fierce critic of Beijing.
The verdict comes as the mainland is increasingly cracking down on Hong Kong’s rights and freedoms.
Several other activists were sentenced on Friday for participating in two demonstrations, on 18 August and 31 August 2019.
They include veteran campaigner Martin Lee, 82, and lawyer Margaret Ng, 73, whose sentences were suspended.
Earlier this week, Mr Lai’s Apple Daily newspaper published a handwritten letter by him, sent from prison, which read: “It is our responsibility as journalists to seek justice. As long as we are not blinded by unjust temptations, as long as we do not let evil get its way through us, we are fulfilling our responsibility.”
Mr Lai was sentenced to 12 months for the 18 August demonstration and another eight months for 31 August. However, the judge ordered for the sentences to be served concurrently except for two months. Former lawmaker Lee Cheuk-yan was also jailed for both demonstrations.
The tycoon faces another six charges – two of which were imposed under the country’s new National Security Law, which can carry a maximum term of life in prison. It is possible that prosecutors could file further charges against him.
The law, implemented in Hong Kong by China last year, criminalises secession and subversion. Earlier this month, Beijing overhauled the territory’s electoral rules to ensure more loyalty to the mainland.
Some of the most long-standing and well-known activists were in the dock today. Except Jimmy Lai, all of them were elected legislators.
At 82, Martin Lee was the oldest defendant. Often called as the “father of democracy” in Hong Kong, he is also the most senior barrister in the city.
The court was filled with their families, friends and supporters. Before the hearing started, some in the audience stood up, waved to the defendants and gave them a thumbs-up.
Jimmy Lai appeared to be calm during sentencing, even though he had been charged with two additional charges earlier today.
But one of the most striking parts of the hearing came in the speech delivered by Margaret Ng, a barrister who served as a lawmaker representing the legal profession for nearly two decades, after discharging her lawyer during mitigation.
Adapting a quote from Thomas More who was executed by King Henry VIII, she said: “I stand the law’s good servant but the people’s first. For the law must serve the people, not the people the law.”
There was a round of applause from the audience after her speech.
During sentencing, the judge said: “Actions have consequences for everyone irrespective of who they are.”
Five of the defendants have to be sent to jail as they didn’t get suspended sentences. When they left the dock, many in the audience waved and cried out, “Stay strong!”
The sentencing is part of a series of trials all relating to the large-scale pro-democracy protests two years ago.
In 2019, protests culminated in the most tense wave of demonstrations in years, often ending in widespread violence between police and activists.
The pro-democracy campaigners were already found guilty of unauthorised assembly earlier this month.
Their defence team had argued that freedom of assembly is protected under Hong Kong’s constitution, and that authorities had approved a demonstration which only then grew into the unauthorised march.
The prosecution argued that freedom of assembly – while granted in the constitution – was not absolute in Hong Kong.
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