Eight days after his election as Bar chairman on 21 January 1999, the Court of Final Appeal ruled that mainland Chinese children born before their parents became Hong Kong permanent residents were entitled to right of abode in the city. In June 1999, the National People's Congress Standing Committee (NPCSC) made an interpretation of the Basic Law that effectively overruled the city's top court in the case. Tong opposed the NPCSC's interpretation, warning that a "Damocles sword" was hanging over the head of the Court of Final Appeal as a result of the government's refusal to rule out requesting Beijing to interpret the law in future cases. He said the failure to make a public promise not to seek further interpretations of the Basic Law from Beijing had damaged public confidence in the rule of law. "Confidence in our legal system and the independence of our judiciary are bound to suffer," he said in his annual report to barristers.
Tong also targeted then Secretary for Justice Elsie Leung Oi-sie's handling of the Sally Aw Sian case, in which the publishing tycoon was not prosecuted for a fraud plot involving her company although she was named as a conspirator in the charges.
Tong sat as a Deputy High Court Judge in 2002.
He ran in the 2002 Election Committee Subsector by-elections in the Legal sub-sector, which was responsible for electing the Chief Executive of Hong Kong in the 2002 election.
In 2002, he co-founded the Article 23 Concern Group with former Bar Association chairmen Audrey Eu Yuet-mee and Alan Leong Kah-kit, to oppose the government's attempt t
In 2002, he co-founded the Article 23 Concern Group with former Bar Association chairmen Audrey Eu Yuet-mee and Alan Leong Kah-kit, to oppose the government's attempt to implement Article 23 of the Basic Law, which they believed posed a threat to civil liberties and basic freedoms. He entered the spotlight as a legal expert when half a million Hong Kong people took to the streets in 2003 to protest against the proposed Article 23 anti-subversion bill that was later shelved. After 1 July protest, the group transformed into the Article 45 Concern Group to call for universal suffrage in 2007 and 2008, as required under Article 45 of the Basic Law .
In the 2004 Legislative Council election, he and fellow barristers from the group Audrey Eu Yuet-mee and Alan Leong Kah-kit ran for the geographical constituency direct elections. Tong combined with the other pro-democrats with a joint list in the New Territories East, where he was placed behind the Democratic Party's Andrew Cheng Kar-foo and The Frontier's Emily Lau Wai-hing. The list received more than 160,000 votes which Cheng, Lau and Tong were elected.
In March 2006, he and members of the Article 45 Concern Group co-founded the Civic Party and he became a member of the party's executive committee. In the 2008 and Civic Party and he became a member of the party's executive committee. In the 2008 and 2012 Legislative Council elections, he was re-elected to represent New Territories East.
In the 2011 District Council election, he ran in the City One hoping for entering the new District Council (Second) constituency race created under the 2012 constitutional reform package but was defeated by pro-Beijing independent Wong Ka-wing.
As a moderate pan-democrat, Tong opposed the party's decision in January 2010 to join the "Five Constituencies Referendum", in which five democratic legislators, representative of the bloc, resigned and re-stood in their constituencies as a de facto referendum over the 2012 constitutional reform package, an action that was heavily criticised by Beijing. Tong intended to vote for the modified reform package, but was required to vote with the rest of Civic Party to oppose it.
2015 electoral reform
During the debate over the electoral reform over the 2017 Chief Executive election, Tong publicly criticised as unreasonable the pan-democrats' support of party or public nomination for chief executive candidates. He put forward a more moderate proposal in October 2013. The proposal suggested increasing the membership of the nominating committee from the 1,200-member Election Committee to 1,514, while maintaining the nomination threshold of 150 votes. Tong recommended the instant runoff voting system, which is used in Ireland, Australia, Sri Lanka; in mayoral elections in London, San Francisco; and in elections for some state governors in the United States, to elect a CE who could be acceptable to all sectors. Tong also advocated repealing an existing law which disallows the Chief Executive belonging to a political party membership.
On 31 August 2014, when Beijing announced its decision constraining Hong Kong's political reform, which would spark the 2014 Hong Kong protests, Tong was immediately critical, vowing to vote against it, which cast severe doubt on the government's ability to win the two-thirds majority a reform package needed in the Legislative Council.
On 31 August 2014, when Beijing announced its decision constraining Hong Kong's political reform, which would spark the 2014 Hong Kong protests, Tong was immediately critical, vowing to vote against it, which cast severe doubt on the government's ability to win the two-thirds majority a reform package needed in the Legislative Council. He cried as he reacted on a live Cable TV programme. "It is the darkest day in the road for democracy," he said. "I am disheartened … I don't see a future for moderates in Hong Kong politics." A moderate reform plan he drew up – under which the public would not be allowed to nominate chief executive candidates – received a cool response from his allies. The barrister said he would think carefully about the next step in his political career after a decision from Beijing that was "more undemocratic than I could imagine". "I thought there would be [some] chance for future dialogue," he said, referring to when pan-democrats were invited for talks with Beijing officials the previous month. "But now, I don't see any chance." He eventually voted against the unmodified proposal with other pan-democrat legislators. On 8 June 2015, before the vote, he set up a think tank Path of Democracy, composed of moderate democrats.
On 6 December 2014, Tong stepped down from the executive of the Civic Party that he co-founded. On 22 June 2015, a few days after the legislative vote, he announced that he would quit as a member of the party, saying that since the end of 2009, the Civic Party's line had deviated from its founding values. He would also resign from the Legislative Council saying it was inappropriate for him to continue having stood and been elected representing the Civic Party.
2020 Defence of new security law
He appeared on the BBC show "Hard Talk" where he defended the new security law. He said, "I am still fighting for democracy for Hong Kong, but is [sic] no way to fight democracy by trying to call for independence is there? Is there any real chance for anybody in Hong Kong or elsewhere, seriously think that by calling for independence to Hong Kong would succeed in getting full democracy established in Hong Kong?"
He was seen as a supporter of He was seen as a supporter of Carrie Lam in the 2017 Chief Executive election. After the election, he was appointed by Lam to the Executive Council of Hong Kong (ExCo), being the only non-official member in the ExCo who came from pro-democracy background.