“My destination: London,” Law wrote in a Facebook post on Monday, his 27th birthday.
“Thus far I’ve kept a low profile on my whereabouts in order to mitigate the risks,” he added.
“In this strange land, I began planning for the life ahead of me. There remains so many uncertainties,” he wrote.
Law was a founding member of Demosisto, a pro-democracy party that disbanded the same day Beijing imposed its new security law on the semi-autonomous business hub.
The party campaigned for democracy and for Hong Kongers to have a greater say in how the city is run but they did not advocate independence.
Nevertheless, Law and other prominent party members such as former student leader Joshua Wong were vilified by Beijing, often described as “black hands” and separatists who conspired with foreigners to undermine China.
Law and Wong both became household names as student leaders during democracy protests in 2014.
Two days after Beijing’s new security law was imposed on the city, Law announced he had left for good but kept his destination secret.
A day before he had given video evidence to a congressional hearing in Washington during which he shouted the popular protest slogan “Liberate Hong Kong. Revolution of our times.”
Authorities say that slogan now contravenes the new security law.
Beijing’s legislation bypassed Hong Kong’s legislature and its contents were kept secret until the law was enacted.
It targets subversion, sedition, terrorism and foreign collusion with up to life in prison.
But its broad phrasing — such as a ban on encouraging hatred towards China’s government — has sent fear rippling through a city used to being able to speak its mind.
Under the law China has claimed jurisdiction over some serious cases and allowed its intelligence apparatus to set up shop openly in the city for the first time.
On the mainland, national security laws are routinely used to crush dissent.
China says the law is needed to return stability after huge and often violent pro-democracy protests last year.
Britain views the security law as a breach of Beijing’s agreement to let Hong Kong maintain key civil liberties — as well as legislative and judicial autonomy — until 2047.
London has infuriated China by announcing plans to grant expanded immigration rights to some three million Hong Kongers eligible for British Overseas National status.
China has vowed some to take measures in response.