It's the latest legal setback for Wong, who has been involved in multiple court cases in the aftermath of the protests, including one that also resulted in a prison sentence, which he's appealing.
Joshua Wong, the youngest and best known leader of the so-called "Umbrella Revolution", would be going back to prison for failing to clear an area in Hong Kong's Mong Kok district that had been taken over by protesters in November 2014, Efe news reported.
Judge Andrew Chan described Wong's involvement in obstructing the clearance in 2014 as "deep and extensive" in his written judgement.
The High Court also gave suspended sentences of 12 or 18 months to 14 others, some of whom such as Lester Shum were also fined between HK$10,000 (around US$1,280) and HK$15,000.More news: Falling US crude stocks, threats to Nigerian fuel infrastructure push oil up
Leaders of the 79-day civil disobedience movement rejected a August 31, 2014 decree from China's parliament, the National People's Congress (NPC), which required the vetting of candidates for the city's top job by a pro-Beijing committee, as "fake universal suffrage".
The remainder of the activists were handed suspended jail terms, including former Occupy Central student leader Lester Shum.
The umbrellas carried by the protesters became a symbol for the democracy movement in Hong Kong. Both were denied bail. "The only appropriate punishment for (Wong) will be one of immediate imprisonment".
He added that instead of prosecuting protestors, the government should bear its constitutional responsibility, while Beijing "should honor its promise of allowing Hong Kong universal suffrage".More news: Man suffered heart attack after missile alert
While a spirited young generation has given Hong Kong's flagging push for democracy fresh impetus, some 100 key activists have, or could be, jailed in upcoming trials, in what critics say is an attempt to counter the movement's momentum.
Beijing has been further incensed by the emergence of some activists calling for independence for Hong Kong since the failure of the Umbrella Movement to win reform.
The by-elections will be held on March 11 to fill four seats vacated by lawmakers disqualified over their oaths of office.
Before entering the court, Wong had told reporters that he would calmly face any sentence. "However, it may well deter others from participating", he adds, by reinforcing the message that "the pro-democracy camp is not welcome".More news: Rogue police officer often known as Venezuelan Rambo is cornered by authorities