Wong and two others climbed over a fence into the forecourt of the Hong Kong government complex on 26 September 2014.
Their arrest helped trigger mass pro-democracy rallies that occupied parts of the city for more than two months.
The movement called on Beijing to allow fully free elections for the leader of the semi-autonomous territory.
Many in Hong Kong feel the chief-executive - currently chosen by a 1,200-member election committee largely viewed as pro-Beijing - should be chosen by a democratic vote.
In August 2014, the Chinese government said it would allow the chief executive to be elected by the public - but that only pre-approved candidates would be eligible to stand.
This sparked the mass street protests and sit-ins, that became known as the Occupy Central movement.
In mid-December of 2014 police dismantled the main occupation sites with the pro-democracy side winning no concessions from Beijing.
Hong Kong's democracy debate
Wong, 19, became the face of the movement, which was driven in large part by youth in Hong Kong. He faces up to two years in prison.
He was cleared of a second charge of incitement.
Student activist Alex Chow was also found guilty of "taking part in an unlawful assembly" and Nathan Law was found guilty of "inciting others to take part in an unlawful assembly".
All three were released on bail to be sentenced on 15 August.
Speaking outside the court house, Wong told reporters he had been prepared for the verdict and that the fight for democracy was a long-term battle.
Wong and Law are now leaders of a new political party, Demosisto, but if they are imprisoned this could well end their hopes of taking part in elections to the Hong Kong legislature in September, BBC Chinese's Martin Yip in Hong Kong says.
A court in Hong Kong has found student activist Joshua Wong guilty of taking part in an unlawful assembly.