While in college, she served as an officer of the 34th Association of Public Administration Disciplines and became the external vice president of the Interim Committee of the City University of Hong Kong Student Union on 3 June 2019.

[7] On 6 November 2019, Siu accepted an exclusive interview with Deutsche Welle's Conflict Zone titled "Hong Kong: Will violence kill the pro-democracy movement?"

Later that month, she announced her withdrawal from the HKIAD following the passing of the Hong Kong National Security Law in order to avoid tainting her relatives and friends due to her political stance.

[17] On 6 April 2022, she spoke at the Summit for Human Rights and Democracy in Geneva, Switzerland, and on 18 October 2022, at the Oslo Freedom Forum in Taiwan, she stated that her "situation is to let everyone know how dangerous the national security law is.

"[18][19] In December 2023, Hong Kong police offered HKD 1 million (US$128,000) bounties for information leading to the capture of five prominent democracy activists based abroad including Siu and wanted for national security crimes.

[23] As of 2024, she serves as a program associate at the National Democratic Institute[24] and an ombudsman of the World Liberty Congress, a global movement of pro-democracy leaders.