From 2004 to 2008 he served as a member for the Medical functional constituency, losing in the 2008 Hong Kong legislative election to Leung Ka-lau.

Kwok sharply criticized this decision, saying that the risk of imported COVID-19 cases from mainland China was still great, and likening the step to "inviting a wolf into your home".

[3] Five weeks ahead of the (subsequently postponed) 2020 Hong Kong Legislative Council Election, on 30 July 2020, as Kwok prepared to defend his seat, the government stated that he was among a dozen pro-democracy candidates whose nominations were 'invalid', under an opaque process in which, nominally, civil servants – returning officers – assess whether, for instance, a candidate had objected to the enactment of the national security law, or was sincere in statements made disavowing separatism.

[11] On 6 January 2021, Kwok was among 53 members of the pro-democratic camp who were arrested under the national security law, specifically its provision regarding alleged subversion.

Kwok called the decision "extremely ridiculous" and asked Chief Executive Carrie Lam to request an explanation from Macau authorities.