Despite that promise, Hong Kong's agency has been tested, reinforced and challenged again in the succeeding years, most recently this week, which witnessed what may have been the largest protest since Hong Kong became Chinese territory again.
Here's a look at key events in Hong Kong people's fight to determine their future:
2003: NATIONAL SECURITY
The first major ripple came when Hong Kong's leaders introduced legislation that would forbid acts of treason and subversion against the Chinese government. The bill resembled laws used to charge dissidents on the mainland, and also banned foreign political entities from conducting political activities and establishing relationships with political groups in Hong Kong.
It sparked one of the three largest protests of the post-1997 era. Organizers estimated that half a million people turned out against the bill, which they saw as an affront to Hong Kong's autonomy and rule of law. As a result of the backlash, a member of the chief executive's council resigned in protest and further action on the proposal was halted.
2010: DIRECT ELECTIONS
The Basic Law states that the ultimate aim is for Hong Kong voters to achieve a complete democracy, but 10 years after the handover, China decided in 2007 that universal suffrage in elections of the chief executive could not be implemented until 2017. Some lawmakers are chosen by business and trade groups, while others are elected by vote.
In a bid to accelerate a decision on universal suffrage, five lawmakers resigned. But this act was followed by the adoption of Beijing-backed electoral changes which expanded the chief executive's selection committee and added more seats for lawmakers elected by direct vote. The legislation divided Hong Kong's pro-democracy camp, as some supported the reforms while others said it would only delay full democracy while reinforcing a structure that favors Beijing. It nevertheless marked the first changes to the electoral system since the handover.
2014: UMBRELLA REVOLUTION
Harking back to its promise to allow Hong Kong residents to vote for their leader in 2017, the Chinese government introduced a bill allowing that, but with one major caveat: the candidates must be approved by Beijing. "The Chief Executive shall be a person who loves the country (China) and loves Hong Kong," read the decision by China's National People's Congress.
Pro-democracy lawmakers were incensed by the bill, which they called an example of "fake universal suffrage" and "fake democracy."
The move triggered a massive protest of a different kind as crowds occupied some of Hong Kong's most crowded districts for 70 days. The movement's organizers called it "Occupy Central with Love and Peace," but it came to be known around the world as the "Umbrella Revolution" for the yellow umbrellas that protesters used as shields against police pepper spray.
In June 2015, Hong Kong legislators formally rejected the bill, and electoral reform has been stalled since then. The current chief executive, Carrie Lam, widely seen as the Chinese Communist Party's favored candidate, was chosen in 2017 by a 1,200-person committee dominated by pro-Beijing elites.
2019: THE EXTRADITION BILLS
Lam is pushing forward amendments to extradition laws that would allow crime suspects to be sent to mainland China to face trials. The proposed legislation triggered a huge protest on Sunday, with organizers putting the turnout at 1 million, and a standoff involving thousands on Wednesday that forced the legislature to postpone debate on the bills.
Unease has been growing following a series of events which pointed to Beijing's growing influence on the region, and some residents say this may be their last chance to speak out publicly without threat of arrest at the request of mainland authorities.
Last year, a Hong Kong-based bookseller who had been released from prison in China was seized by Chinese agents while traveling on a train in January with Swedish diplomats. Gui Minhai, a Swedish citizen, sold gossipy books about Chinese leaders and was one of five Hong Kong booksellers who wound up in Chinese custody in 2015.
Then in September, Hong Kong banned the Hong Kong National Party, which advocates independence for the territory, on national security grounds. Shortly after, it denied a visa renewal for Hong Kong-based Financial Times editor Victor Mallet, who had introduced the leader of the party at a Foreign Correspondents' Club event in August.
Lam says the bill will include safeguards to protect human rights, but opponents have decried its potential to erode Hong Kong's rule of law and judicial independence.