Chung was arrested in October 2020 by police in plainclothes reportedly while he was attempting to seek asylum at the US consulate in Hong Kong. Since then he remained in detention on remand.
The student activist said he pleaded guilty with “no shame in his heart,” appearing in court about three weeks ago, reported the Hong Kong Free Press.
The secession charges are related to an activist group called Studentlocalism he established as a student but disbanded it before Beijing imposed the security law in June 2020 to prosecute those involved in subversion, secession, terrorism and collusion with foreign forces.
He was also charged for his social media posts and possession of pro-independent materials which has been made illegal under the national security law.
Chung, however, pleaded not guilty on two other charges, including a sedition charge of conspiracy of publishing seditious content and one other count of money laundering.
The charges were filed under a plea bargain deal which led to a reduction of 25 per cent in his sentence.
Court Judge Stanley Chan said: "He actively organised, planned and implemented activities to separate the country.” He said even when his group Studentlocalism was disbanded, he “continued his endeavour and the pursuit of his political ideas” through its US-based division.
Prosecutor Ivan Cheung said that Chung was merely the administrator for the social media pages of the US branch of Studentlocalism.
Human rights organisation Hong Kong Watch’s policy director Johnny Patterson said the court’s decision was "disproportionate, draconian, and sets a dangerous precedent for other young Hong Kongers whose only crime is using social media to protest the dismantling of Hong Kong’s freedoms."
"At 20 years old, Tony Chung is the youngest person to be sentenced under this draconian law. He will not be the last," Mr Patterson added.
Eric Lai Yan-ho, a law fellow at Georgetown University’s Centre for Asian Law, said in Hong Kong an environment has been created where “defendants are pressured to plead guilty rather than defend themselves.”
“The governments should ask themselves whether criminalising young people with draconian laws can really win back public trust toward the court or the government,” Mr Lai said in a tweet.
Evidence such as pro-independence T-shirts, flags and books were seized from Chung’s home in a police raid in 2020 were presented in court. A PayPal account to receive donation was also presented as evidence in money laundering charges.
Chung received the most lenient sentence of the three people convicted so far under the law.
Tong Ying-kit and Ma Chun-man, who has pled not guilty, received 9 years and 5.9 years in jail respectively.
The latest sentence has added to the existing concerns over the sweeping law which analysts say is being used to crack down on political opposition and gain more control over the former British colony.