Tsui graduated in social sciences from the University of Ottawa and holds a Master of Business Administration degree from the University of Manchester. He worked at several banks including HSBC in Canada, Bank of Montreal, Merrill Lynch and American Express Bank before becoming an investment services manager at Hang Seng Bank. He stepped into politics in 2006 when he joined the Democratic Alliance for the Betterment and Progress of Hong Kong (DAB), the largest pro-Beijing party in Hong Kong. He was the vice-chairman of its youth branch, the Young DAB, from 2007 to 2008.
He joined the government in the appointed role of Political Assistant to the Secretary for Home Affairs in 2008, after he renounced his Canadian citizenship that year. In 2017 he became Under Secretary for Labour and Welfare. In April 2020, he was appointed Secretary for Home Affairs, while remaining a member of the DAB, succeeding Lau Kong-wah.
- "New faces Carrie Lam has enlisted to help Hong Kong tackle coronavirus impact". South China Morning Post. 20 April 2020.
- "Government announces appointment and removal of Principal Officials (with photos)". Hong Kong Government. 20 April 2020.
|New title|| Political Assistant to the Secretary for Home Affairs
| Under Secretary for Labour and Welfare
| Secretary for Home Affairs